LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 

Deceive J 
Accessions  Na7~.  O,m  No. 


QUANTITATIVE 

CHEMICAL    ANALYSIS 


BY 


ELECTROLYSIS. 


BY 

DR.    ALEXANDER  CLASSEN, 

PROFESSOR  OP  CHEMISTRY  AND   DIRECTOR  OF  THE   INORGANIC   LABORATORY  IN  THE 
ROYAL  SCHOOL  OF  TECHNOLOGY  AT  AACHEN. 


®ranslation> 


SECOND  ENGLISH.  FROM  THE  THIRD  GERMAN  EDITION, 
REVISED  AND  GREATLY  ENLARGED. 

BY 

WILLIAM   HALE   HERRIOK,   A.M., 

FORMERLY  PROFESSOR   OF  CHEMISTRY   IN  IOWA   COLLEGE,   AND   IN  THE 
PENNSYLVANIA  STATE  COLLEGE. 


SECOND  THOUSAND. 


NEW  YORK: 

JOHN    WILEY    &    SONS, 

53  EAST  TENTH  STREET. 

1894. 


COPYRIGHT,  1887,  1894, 
BY  WILLIAM  HALE  HERRICK. 


TRANSLATOR'S  PREFACE  TO  FIRST  EDITION. 


THE  attention  of  the  translator  was  drawn  to  the 
original  work,  of  which  the  following  is  a  translation, 
by  finding  it  to  be  the  only  source  of  knowledge  of  the 
subject  outside  of  scattered  articles  in  the  journals,  and 
thus  convenient  and  almost  indispensable,  as  a  laboratory 
handbook,  to  himself  and  advanced  students. 

It  is  in  the  hope  of  rendering  it  more  available  to 
all  who  may  have  occasion  to  use  electrolytic  methods  in 
quantitative  analysis,  and  of  increasing  and  stimulating 
the  use  of  these  valuable  methods,  that  he  has  undertaken 
the  translation. 

Twelve  or  fifteen  years  ago,  when  the  translator  was 
a  student,  electrolytic  methods  in  analysis  were  practically 
unknown  in  one  of  the  first  laboratories  of  the  country ; 
they  still  occupy  a  very  subordinate  position  in  many 
of  the  leading  laboratories.  The  translator  believes  that 
they  can,  with  advantage,  be  much  more  widely  used  in 
this  country,  in  both  scientific  and  technical  laboratories, 
especially  where  a  current  from  a  dynamo,  or  power  to 
run  a  small  dynamo,  is  available.  He  trusts  that  he 
may  be  found  to  have  contributed  to  such  a  result  by 

iii 


iv  TRANSLATOR'S    PREFACE. 

rendering  available,  in  English,  this  complete  and  standard 
work. 

As  the  original  is  the  work  of  a  specialist,  and  recently 
published,  the  translator  has  had  little  occasion  to  add  to 
his  labors  those  of  .  an  .editor.  A  few  .additions  have 
been  made,  touching  matters  of  more  recent  date  that 
have  fallen  under  his  eye.  All  such  additions  are  enclosed 
in  brackets. 

The  thanks  of  the  translator  are  due  to  Prof.  O.  D. 
Allen,  of  the  Sheffield  Scientific  School  of  Yale  Colleger 
for  revision  of  portions  of  the  translation  relating  to  the 
analysis  of  metallurgical  products ;  and  to  his  colleagues,. 
Profs.  I.  Thornton  Osmond  and  William  Frear,  —  to  the 
former  for  revision  of  the  portion  of  the  proof  relating 
to  the  use  of  the  dynamo,  and  to  the  latter  for  frequent 
suggestions  throughout  the  progress  of  the  work. 


WILLIAM  HALE  HERRICK. 


THE  PENNSYLVANIA  STATE  COLLEGE, 
CHEMICAL  LABORATORY, 

June,  1887. 


PREFACE  TO  SECOND  EDITION. 


SINCE  the  appearance  of  the  first  edition  of  this  book, 
I  have  devoted  myself  exclusively  to  experimental  work 
having  for  its  object  the  further  development  of  quantitative 
analysis  by  electrolysis  as  an  independent  branch  of  the 
subject.  This  new  method  of  analysis  may  now  be 
considered  as  established  in  its  essential  points. 

The  great  advantage  of  quantitative  electrolysis,  apart 
from  its  greater  simplicity,  lies  unquestionably  in  the  fact 
that  the  electric  current  does  the  work  of  the  analyst, 
setting  him  free  to  carry  on  other  work. 

Long  experience  has  shown,  that,  if  the  methods  are 
correctly  followed,  even  unskilled  analysts  obtain  results 
that  experienced  chemists  can  with  difficulty  reach  by  the 
ordinary  methods  of  gravimetric  analysis. 

Since  a  large  number  of  the  most  different  analyses 
can  now  be  carried  on  at  the  same  time  (a  result 
hitherto  impossible  to  attain),  I  may  be  permitted  to 
hope  that  analysis  by  electrolytic  methods  may  come 
more  and  more  into  use  in  scientific  and  technical 
laboratories. 

A.  CLASSED 

AACHEN,    September,   1885. 


TRANSLATOR'S  PREFACE  TO  SECOND  EDITION. 


LITTLE  needs  to  be  said  in  introducing  this  new  edition. 
The  author,  in  his  preface,  calls  attention  to  the  many  addi- 
tions that  have  been  made.  It  will  be  seen  that  the  majority 
of  these  additions  record  the  work  of  others  than  the  author, 
so  that  while  the  earlier  editions  were  largely  a  record  of  the 
work  of  the  Aachen  school,  this  edition  is  rather  a  compend 
of  the  literature  of  the  subject.  This  fact  alone  is  sufficient 
evidence  of  the  great  advance,  within  seven  years,  in  the  use 
and  development  of  electrolytic  methods. 

As  before,  it  has  not  seemed  wise  to  add  much  new  mat- 
ter. Thanks  are  hereby  rendered  to  Professor  Edgar  F. 
Smith,  our  leading  American  worker  in  this  field,  for  free 
permission  to  use  his  "  Electro-chemical  Analysis."  It  has 
not  seemed  best,  however,  to  transfer  methods  from  Professor 
Smith's  book,  as  practical  workers  would  doubtless  have  access 
to  both  books,  and  would  prefer  to  consult  the  original.  For 
the  same  reason,  no  attempt  has  been  made  to  summarize 
periodical  literature  on  this  subject,  for  the  two  years  since 
the  publication  of  the  German  edition. 

Thanks  are  due  to  Dr.  Leonard  Paget,  for  placing  at  the 
disposal  of  the  translator  and  users  of  this  book  his  simple 
and  efficient  thermopile. 

WILLIAM   HALE   HEREICK. 
NEW  YORK,  August,  1894. 

vi 


PREFACE  TO  THIRD  EDITION. 


IN  the  present  edition  will  be  found  a  large  number  of 
new  methods  and  improvements. 

In  the  preface  to  the  second  edition  I  expressed  the  hope 
that  electrolytic  analysis  would  find  increasing  use  in  both 
.scientific  and  technical  laboratories.  The  methods  of  elec- 
trolysis are  to-day  taught  as  special  methods  of  quantitative 
analysis,  like  gas  and  spectroscopic  analysis,  in  most  of  the 
laboratories  of  the  higher  institutions  of  learning  in  this  and 
•other  countries  (this  book  has  been  translated  into  French  by 
Professor  Bias  and  into  English  by  Professor  Herrick),  and 
are  employed  for  scientific  researches,  as  well  as  for  the  pro- 
duction of  absolutely  pure  metals  and  determinations  of  atomic 
weight. 

It  fills  me  with  satisfaction  that  the  introduction  of  elec- 
trolytic analysis  into  the  laboratories  of  the  great  industries 
has  made  such  progress;  in  some  such  laboratories  two  or 
three  thousand  determinations  are  made  per  year. 

The  statements  of  various  associations  of  metal-workers, 
large  foundry-owners,  etc.,  agree  in  emphasizing  the  decided 
advantages  of  electrolytic  methods  over  the  gravimetric 
methods  hitherto  in  use,  and  their  indispensability  in  certain 

cases.     The  special  advantages  of  these  methods,  according  to 

vii 


Vlll  PREFACE   TO   THIRD    EDITION. 

the  above-mentioned  testimony,  are  accuracy  and  rapidity. 
These  two  factors  have  made  it  possible  to  utilize  electrolysis 
for  complete  and  exact  control  of  certain  metallurgical  opera- 
tions, and,  as  more  fully  stated  elsewhere  in  this  book,  for  the 
purchase  of  ores  in  foreign  markets;  results  often  impossible 
by  the  use  of  previous  gravimetric  methods. 

A.  CLASSEN. 
AACHEN,  July,  1892. 


CONTENTS. 


PAKT  I.— GENERAL  PART 
INTRODUCTION. 

PAGE 

GALVANIC  BATTERIES 7 

THE  LECLANCHE  CELL 8 

THE  MEIDINGER  CELL 9 

THE  DANIELL  CELL 11 

THE  GROVE  CELL 14 

THE  BUNSEN  CELL 15 

THERMO-ELECTRIC  PILES 16 

CLAMOND  THERMOPILE 16 

NOE  THERMOPILE 18 

PAGET  THERMOPILE 21 

ELECTRICAL  MACHINES 24 

LABORATORY  DYNAMO  AND  ACCOMPANYING  APPARATUS  FOR  ELECTRO- 
LYTIC PURPOSES 25 

SECONDARY  BATTERIES 37 

THE  USE  OF  ACCUMULATORS,  AND  THEIR  ADVANTAGES  OVER  GAL- 
VANIC BATTERIES 40 

APPARATUS    FOR    REDUCING  THE  STRENGTH    OF    THE    CURRENT. — 

RESISTANCES 48 

MEASUREMENT  OF  THE  STRENGTH  OF  THE  CURRENT    57 

PROCESS  OF  ANALYSIS. — ELECTRODES,  SUPPORTS,  ETC 64 

GRAVIMETRIC  DETERMINATION  OF  METALS. 

DETERMINATION  OF  IRON 78 

"  COBALT 81 

"  NICKEL 82 

ix 


X  CONTENTS. 

PAGE 

DETERMINATION  OF  ZINC 83 

"  "MANGANESE 85 

"  "  ALUMINIUM,  CHROMIUM,  URANIUM,  BERYLLIUM.     8? 

"  "COPPER , 88 

"  "  BISMUTH 91 

"CADMIUM 94 

"LEAD 96 

••  "  THALLIUM 99 

"  "  SILVER 100 

"  "MERCURY 102 

"PLATINUM     104 

"  "  PALLADIUM 106 

"  GOLD 106 

"  ANTIMONY 106 

"  TIN 110 

"ARSENIC 113 

"  "  POTASSIUM,  AMMONIA  (NITROGEN) 113 

"  "  NITRIC  ACID  IN  NITRATES.  .  .113 


SEPARATION  OF  METALS. 

IRON  AND  COBALT 115 

"      "    NICKEL  115 

"    FROM  COBALT  AND  NICKEL  116 

"    AND  ZINC 116 

IRON,  COBALT,  NICKEL,  AND  ZINC,  FROM  ALUMINIUM  — 117 

IRON  FROM  MANGANESE 118 

NICKEL  FROM  MANGANESE 121 

COBALT  AND  ZINC  FROM  MANGANESE 1 22 

NICKEL,  COPPER,  CADMIUM,  ZINC,  AND  MERCURY,  FROM  MANGANESE  122 

MANGANESE  FROM  COPPER,  CADMIUM,  MERCURY 123 

IRON,   COBALT,    NICKEL,   AND  ZINC,   FROM  MANGANESE  AND  ALU- 
MINIUM  123 

IRON,  COBALT,  NICKEL,  AND  ZINC,  FROM  CHROMIUM 123 

"  "  "  "       "         "*  "         AND  ALUMINIUM.  124 

IRON,  COBALT,  NICKEL,  AND  ZINC,  FROM  MANGANESE,  CHROMIUM,  AND 

ALUMINIUM 124 

IRON,  COBALT,  NICKEL,  AND  ZINC,  FROM  URANIUM 125 

"  "  "       "         "      CHROMIUM  AND  URANIUM.  .  125 

IRON,  NICKEL,  COBALT,  AND  ZINC,  FROM  ALUMINIUM,  MAGNESIUM, 

AND  URANIUM 126 

MANGANESE  FROM  BARIUM,  STRONTIUM,  CALCIUM,  MAGNESIUM,  AND 
ALKALIES  : . .   126 


CONTENTS.  XI 

PAGE 

IRON  FROM  BERYLLIUM 12$ 

"       "  "  AND  ALUMINIUM 127 

"       "      ZIRCON 127 

"       "      VANADIUM  , 127 

"       "      MANGANESE  AND  PHOSPHORIC  ACID    127 

*'       "      MANGANESE,  ALUMINIUM,  AND  PHOSPHORIC  ACID 128 

"       "  "  AND  SULPHURIC  ACID 129 

COPPER  FROM  BISMUTH . .  129 

"  "      CADMIUM  130 

"      LEAD 180 

"  "      SILVER 130 

"          "      ANTIMONY  AND  ARSENIC 131 

"      TIN... 138 

COPPER  FROM  IRON,  COBALT,  NICKEL,  ZINC,  MANGANESE,  CHROMIUM, 

ALUMINIUM,  AND  PHOSPHORIC  ACID 133 

COPPER  FROM  BARIUM,  STRONTIUM,  CALCIUM,  POTASSIUM,  SODIUM, 

AND  LITHIUM 133 

BISMUTH  FROM  IRON,  NICKEL,  COBALT,  ZINC.  MANGANESE,  CADMIUM, 

CHROMIUM.  ALUMINIUM,  AND  URANIUM 134 

LEAD  FROM  CADMIUM 134 

•'        "      BISMUTH 134 

"        "      SILVER 134 

"      MERCURY 135 

LEAD  FROM  IRON,    COBALT,   NICKEL,  ZINC,  CHROMIUM,  AND  ALU- 
MINIUM  135 

CADMIUM  FROM  ZINC  ....  1 35 

NICKEL  AND  COBALT 137 

CADMIUM  AND  BISMUTH  FROM  MANGANESE,   CHROMIUM,  AND  ALU- 
MINIUM    1 37 

MERCURY  FROM  SILVER 137 

"  "      COPPER 137 

"     ARSENIC  AND  PALLADIUM 138 

MERCURY  FROM  IRON,  COBALT,  NICKEL,  ZINC,   MANGANESE,   CHRO- 
MIUM, AND  ALUMINIUM 138 

ANTIMONY  FROM  TIN  138 

"  "      ARSENIC 140 

ANTIMONY,  ARSENIC,  AND  TIN 141 

TIN  FROM  PHOSPHORIC  ACID 144 

PLATINUM  FROM  IKIDIUM 144 

SEPARATION  OF  GOLD  FROM  OTHER  METALS 144 

POTASSIUM  FROM  SODIUM 145 

SODIUM  AND  AMMONIA 145 


Xll  CONTENTS. 


PART  II.— SPECIAL  PART. 

PAGE 

ALLOY  OF  COPPER  AND  ZINC  [LEAD,  IRON]  (BRASS) 14? 

"     SILVER  (SILVER  COIN) 148 

"       "  TIN  AND  LEAD  (SOLDER) 149 

"       "  LEAD  AND  BISMUTH . . 149 

"      "       "    ZINC ..  149 

"       "  BISMUTH  AND  COPPER 150 

"  COPPER  AND  TIN  (BRONZE) 150 

"  COPPER,  TIN,  ZINC,  AND  PHOSPHORUS  (Piiospiioit  BRONZE)  131 
ALLOY  OF  COPPER,  TIN,  ZINC,  MANGANESE,  AND  PH  SPHORUS  (M  AN- 

GANESE-PHOSPHOR  BRONZE) 1  51 

ALLOY  OF  NICKEL  AND  COPPER  (NICKEL  COIN)     151 

"  COPPER,  ZINC,  AND  NICKEL  (GERMAN  SILVER) 152 

"  TIN,  LEAD,  BISMUTH,  AND  CADMIUM  (WOOD'S  METAL).  . . .  153 

"       '•  TIN,  LEAD,  BISMUTH,  AND  MERCURY 153 

"  LEAD  AND  ANTIMONY  (HARD  LEAD.     TYPE  METAL; 154 

"       "  ANTIMONY  AND  TIN 154 

"  "          "     ARSENIC    ....  154 

"       "  ANTIMONY,  TIN,  AND  ARSENIC  155 

SPATHIC  IRON 155 

HEMATITE 156 

LTMONITE  t 158 

CLAY  IRONSTONE 158 

BOG  IRON  ORE 158 

CHROME  IRON-ORE  159 

PSILOMELANE 160 

SPHALERITE 163 

CALAMINE  AND  SMITHSONITE 165 

ULTRAMARINE. 165 

REFINERY  SLAG  166 

COPPER  AND  LEAD  SLAGS 1 66 

BLAST  FURNACE,  CUPOLA,  AND  BESSEMER  SLAGS 168 

ZIRCON 169 

ARSENOPYRITE 169 

COPPER  PYRITES 170 

NICKEL  MATTE.     COPPER  MATTE 171 

COPPER  SPEISS.    LEAD  SPEISS 172 

PYRARGYRITE 173 

TETRAHEDRITE 173 

FURNACE  Sows 174 


CONTENTS.  xiii 

PAGE 

STJBNITE  f  ANTIMONY  GLANCE). 175 

ULLMANITE.  . .     . 175 

BOURNONITE 176 

ZlNKENITE 176 

LlNNAEITE 177 

COBALTITE 177 

COBALTIFEROUS  ARSENOPYKITE 178 

CERUSSITE . 179 

GALENA 179 

PYKOMORPHITE '.... 180 

LEAD  MATTE 180 

CINNABAR '.'.... 181 

BlSMUTHINITE ...  181 

URANINITE  (PITCHBLENDE) 182 

SOFT  LEAD .". 184 

HARD  LEAD 186 

ANTIMONY —  ... 187 

SPELTER . ; . . . 187 

BLISTER  COPPER 189 

REFINED  COPPER .  191 

TIN . . . . .  • 192 

BISMUTH 192 

SILVER 193 

NICKEL 198 

PIG  IRON,  STEEL,  SPIEGEL,  FERROMANGANESE J94 


PART  III. 

TABLES  FOR  CALCULATION  OF  ANALYSES 199 

REAGENTS 203 

POTASSIUM  OXALATE  203 

AMMONIUM  OXALATE 203 

OXALIC  ACID 204 

AMMONIUM  SULPHATE 204 

SODIUM  SULPHIDE 204 

ALCOHOL 205- 

ANALYTICAL  RESULTS .    . .  206 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS. 


PAET   L-GESTEEAL   PART. 


INTRODUCTION.* 

WATER  acidified  with  sulphuric  acid  is  decomposed  into 
its  elements,  hydrogen  and  oxygen,  when  a  galvanic  current 
is  passed  through  it;  a  large  number  of  compound  sub- 
stances conduct  themselves  in  a  similar  manner.  This  gal- 
vanic decomposition  is  called  electrolysis,  and  the  substances 
which  are  decomposed  by  the  electric  current  are  known  as 
electrolytes.  The  substances  into  which  electrolytes  are 
separated  by  the  electric  current  are  naturally  divided  into 
two  groups :  Those  which  separate  at  the  positive  electrode, 
or  anode  (connected  with  the  -}-  pole  of  the  source  of  the 
current),  and  which  are  therefore  the  electro-negative  con- 
stituents, are  called  anions;  those  which  separate  at  the 
negative  electrode,  or  kathode  (connected  with  the  —  pole 
of  the  source  of  the  current),  the  electro-positive  constitu- 
ents, are  called  kathions. 

The  metalloids,  or  electro-negative  acid  groups,  therefore 
appear  at  the  positive  electrode,  while  the  metals  are  sepa- 
rated at  the  negative  electrode. 

*  An  elementary  knowledge  of  galvanic  action  is  assumed. 


2  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

For  instance,  if  the  electric  current  is  passed  through 
the  solution  of  a  haloid  salt,  the  halogen  is  separated  at  the 
anode,  the  metal  at  the  kathode. 

CuCl2  =  C12  +  Cu, 

=  C12 


Oxygen  salts  act  in  a  similar  manner. 

CuSO4        =  SO4        +  Cu, 
Cu(N03)2  =  (N03)2  +  Cu. 

Many  acids  are  decomposed  in  a  similar  manner.* 

H2S04  =  SO4  +  H2, 
2HC1    =  C12  +  H2. 

The  substances  formed  by  electrolytic  decomposition, 
however,  generally  undergo  further  chemical  change,  or  are 
acted  on  by  the  electrodes  ;  various  secondary  reactions  take 
place. 

In  the  electrolysis  of  a  solution  of  copper  sulphate  between 
platinum  electrodes,  the  secondary  process  consists  in  the  re- 
action with  water  of  the  group  SO4,  which  cannot  exist 
uncombined. 

SQ4  +  H2O  =  H2S04  +  O. 

T;i  accordance  with  this  reaction,  oxygen  gas  is  given  off  at 
the  positive  pole. 

In  the  electrolysis  of  hydrochloric  acid,  the  chlorine  set 


*  Some  acids  are  not  decomposed  by  the  electric  current;  e.g.,  silicic, 
carbonic,  and  boric  acids. 


INTRODUCTION.  3 

free  at  the  anode  reacts  with  water,  forming  hypochlorous 
acid,  chloric  acid,  perchloric  acid,  etc.  Similar  secondary 
reactions  are  observed  in  the  electrolysis  of  chlorides.  If  a 
solution  of  ammonium  chloride,  for  example,  is  submitted  to 
electrolysis,  the  nascent  chlorine  acts  on  the  undecomposed 
salt,  with  the  production,  among  other  substances,  of  nitro- 
gen, or  nitrogen  chloride.  Haloid  salts  of  the  alkaline  earths 
show  similar  phenomena. 

Nitric  acid  is  decomposed  by  electrolysis  in  accordance 
with  the  re-action 

8HNO3  =  8N02  +  8O  (anode), 

8H  (kathode). 

The  nascent  hydrogen  acts  secondarily  on  nitric  acid : 
8H  +  HN03  =  NH3  +  3H2O. 

In  the  presence  of  sulphuric  acid,  or  a  sulphate,  this 
decomposition  is  complete,  the  final  product  being  ammonium 
sulphate. 

This  decomposition  of  nitric  acid  is  of  practical  impor- 
tance in  chemical  analysis.  From  a  nitric-acid  solution  which 
•contains  copper  and  zinc,  the  former  metal  only  is  reduced ; 
this  fact  can  be  utilized  for  the  separation  of  the  two  metals. 
If,  now,  the  current  is  allowed  to  pass  for  a  long  time  after 
the  reduction  of  the  copper,  the  nitric  acid  is  gradually 
converted  into  ammonia,  and  the  zinc  then  separates  from 
the  solution. 

In  the  electrolysis  of  a  solution  of  lead  nitrate,  a  complete 
secondary  re-action  occurs,  the  ozonized  oxygen  (or  H2O2) 
which  is  formed  at  the  anode  re-acting  on  the  lead  salt  with 
formation  of  lead  peroxide.  If  sufficient  free  nitric  acid  is 


4  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

present,  all  the  lead  separates  as  peroxide  at  the  positive 
electrode. 

Pb(N03)2  =  2N02  +  02  (anode), 

Pb, 
Pb(N03)2  +  02  =  Pb02. 

In  the  electrolysis  of  salts  of  manganese,  bismuth,  etc., 
similar  decompositions  occur. 

If  the  salts  of  metals  which  decompose  water  at  ordinary 
temperatures  (alkalies  and  alkaline  earths)  are  electrolyzed, 
secondary  re-actions  occur  at  the  negative  electrode : 

K2SO4  =  SO4  (anode), 

K2  (kathode), 

(SO4  +  H2O    =  H2SO4  +  O), 
K2      +  2H20  =  2KOH  +  H2. 

The  decomposition  products,  then,  are  sulphuric  acid  and 
oxygen  at  one  electrode,  potassium  hydroxide  and  hydrogen 
at  the  other. 

A  similar  reaction  occurs  in  the  electrolysis  of  chromic 
acid  in  the  presence  of  a  free  acid  : 

2CrO3  =  O6  (anode), 

Cr2  (kathode), 
Cr2  +  3H2O  =  Cr2O3  +  3H2. 

The  metals  disengaged  at  the  negative  electrode  may 
yield  secondary  products  by  acting  on  the  solution.  So,  for 
instance,  in  the  electrolysis  of  cupric  chloride,  the  separated 
copper  reacts  with  the  cupric  chloride  to  form  cuprous 
chloride  ;  copper  acetate  yields,  at  the  kathode,  a  mixture  of 
copper  and  cupric  (or  cuprous)  oxide. 


INTRODUCTION.  5 

In  the  electrolysis  of  organic  compounds,  the  groups  set 
free  at  an  electrode  may  be  decomposed  in  a  manner  analo- 
gous to  that  noted  in  inorganic  compounds,  and  yield  various 
products. 

The  electrolysis  of  potassium  acetate  should  yield,  as  final 
products,  potassium  (potassium  hydroxide)  and  acetic  acid. 

CH3COOK  =  K  +  CH3COO. 

K  -u  CHsCOO  +  H.O  =  KOH  +  CH8COOH. 

Instead  of  this,  the  acetic  acid  splits  either  into  carbon 
dioxide  and  methyl  (dimethyl),  or  ethylene  is  formed  by  the 
action  of  oxygen  on  the  dimethyl. 

Potassium  valerianate  yields,  in  addition  to  valerianic 
acid,  carbon  dioxide  and  dibutyl ;  the  latter  is  oxidized  by 
continued  electrolysis  to  isobutylene  and  water. 

Sodium  succinate  yields,  among  other  products,  ethylene 
and  carbon  dioxide ;  potassium  lactate  breaks  up  into  carbon 
dioxide  and  aldehyde. 

If  a  solution  contains  several  metals,  secondary  reactions 
may  occur  as  follows:  one  of  the  metals  separated  at  the 
negative  electrode  being  more  strongly  electro-positive  than 
the  other  which  is  present  in  the  solution  acts  upon  it,  and 
sets  free  an  equivalent  weight  of  it.  For  instance,  in  the 
electrolysis  of  a  mixture  of  copper  and  zinc  sulphates,  copper 
and  zinc  separate  out  at  the  negative  electrode.  The  latter 
sets  copper  free,  as  follows : 

Zn  +  CuSO4  =  Cu  +  ZnSO4. 

The  elements  separated  by  the  electric  current  sometimes 
recombine,  and  exert  an  electro-motive  force  opposite  to  that 
exerted  by  the  primary  current.  The  electro-motive  force 
thus  produced  is  named  polarization  of  the  electrodes. 

\ 

\ 


6  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Such  counter-currents  may  also  be  produced  by  the  union 
of  gases  disengaged  at  the  anode  and  kathode. 

This  counter-current  may  cause  the  solution  of  the  metal 
separated  at  the  negative  electrode.  So,  in  the  electrolysis- 
of  a  solution  of  copper  sulphate,  it  may  happen,  under  certain 
circumstances,  that  the  free  sulphuric  acid  formed  acts  again 
on  the  copper  to  form  the  sulphate. 

For  the  purposes  of  quantitative  chemical  analysis,  only 
such  solutions  are  adapted,  as  indicated  by  the  foregoing, 
as  are  decomposed  completely  by  the  current  without  the 
formation  of  injurious  intermediate  products.  Solutions 
which  contain  a  free  inorganic  acid  are  well  adapted  to 
electrolysis,  because  they  are  readily  decomposed.  In  the 
presence  of  a  free  acid,  however,  only  a  few  metals  (e.g., 
copper,  mercury,  and  platinum)  separate  completely  ;  so  that 
such  solutions  are  capable  of  only  limited  use. 

Of  all  compounds  of  the  metals,  the  double  oxalates  are 
the  best  adapted  to  quantitative  analysis.*  Oxalic  acid  i& 
decomposed  by  the  electric  current : 

C2H2O4  =  2CO2  (anode), 
H2  (kathode). 

When  potassium  oxalate  is  subjected  to  electrolysis,  the 
principal  decomposition-products  are : 

K2C2O4  =  2CO2  (anode), 

K2  (kathode), 

K2         +  2H2O  =  2KOH  +  H2, 
2KHO  +  2C02  =  2KHCO3. 

*  Classen,  Ber.  d  ch  Ges.,  14,1622.  2771 ;  17,2467  ;  18,1104,  1687  ;  19r 
823  ;  20,  5U-I  ,21,  ^ 


GALVANIC    BATTERIES.  7 

When  ammonium  oxalate  is  used,  the  decomposition- 
products  are  hydrogen  and  hydrogen  ammonium  carbonate. 
The  latter  is  partly  redecomposed  into  ammonia  and  carbon 
dioxide. 

In  the  electrolysis  of  double  oxalates,  e.g.,  of  zinc  ammo- 
nium oxalate,  decomposition  takes  place  as  follows  :  Zinc 
oxalate  breaks  up  into  zinc  and  carbon  dioxide,  and  ammoni- 
um oxalate  into  ammonium  and  carbon  dioxide.  The  carbon 
dioxide,  which  separates  at  the  positive  pole,  combines  with 
the  ammonium  to  form  hydrogen  ammonium  carbonate,  as 
above  explained. 

In  the  decomposition  of  oxalates,  there  are  no  secondary 
reactions  nor  counter-currents  unfavorable  to  the  electrolysis. 
All  oxalates  are  decomposed  by  the  electric  current  with 
greater  or  less  ease,  and  the  reduced  metals  are  not  attacked 
by  the  decomposition-products,  even  when  the  current  be- 
comes weaker  during  the  reaction.  When  the  reaction  is 
complete,  the  solution  can  be  poured  off  at  once,  and  the 
weight  of  the  separated  metal  determined.  (See  further 
details  later.) 

GALVANIC    BATTERIES. 

The  intensity  of  the  current  necessary  for  the  reduction 
of  different  metals  varies  greatly.  Copper,  cadmium,  bis- 
muth, and  platinum,  e.g.,  are  precipitated  from  their  solu- 
tions by  very  weak  currents ;  while  iron,  nickel,  cobalt,  zinc, 
and  other  metals  require  stronger  currents. 

For  the  former,  Meininger,  Daniell,  and  Leclanche'  cells 
are  much  used;  for  stronger  currents,  those  of  Bunsen  and 
Grove. 

The  electro-motive  force  of  these  cells  is  very  unlike  ; 
for  instance,  that  of  a  Daniell  cell  is  1.079  volts,  of  a 


8 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Leclanche*   cell   1.48,   of  a   Bunsen   cell   1.80,   of  a   Grove 
cell  1.81. 

LECLANCHE    CELL. 

This  is  a  one-fluid  cell  using  a  solution  of  ammonium 
chloride,  which  surrounds  the  negative  pole,  the  zinc.     The 


FIG.  1. 

cell  is  much  used  in  the  form  shown  in  Fig.  1.  In  the  jar, 
which  is  square  in  section,  with  a  rounded  projection  at 
one  corner,  stands  a  porous  clay  cup,  from  which  projects  a 
block  of  carbon  K  surrounded  by  coarsely  pulverized  man- 
ganese dioxide,  or  a  mixture  of  manganese  dioxide  and  retort 
carbon.  In  the  projecting  rounded  corner  is  a  stout  rod  Z 
of  amalgamated  zinc.  The  carbon  and  zinc  are  both  provided 
with  binding  screws,  and  are  immersed  in  a  concentrated 
solution  of  ammonium  chloride. 


MEIDINGER    CELL. 


9 


Leclanche*   also   uses,   in   place    of   the   powdered   man- 
ganese dioxide,  compressed  prisms  (shown  in  Fig.  2)  con- 


FIG.  2. 


sisting  of  40  parts  manganese  dioxide,  55  parts  gas  carbon, 
and  5  parts  shellac ;  a  little  potassium  sulphate  is  also 
added  to  increase  the  conductivity.  The  porous  cup  is 
thus  dispensed  with. 


MEIDINGER    CELL. 

In  contrast  to  the  Leclanche'  cell,  that  of  Meidinger  con- 
tains two  liquids,  solutions  of  magnesium  and  copper  sul- 
phates. The  element  is  constructed  as  follows :  In  the  glass 
vessel  G  (Fig.  3)  stands  a  smaller  glass  g,  and  in  this  a 
copper  cylinder  K  to  which  an  insulated  copper  wire  D  is 
fastened. 


10 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


A  second  cylinder  Z  of  zinc,  to  which  the  projecting  wire 
j  is  fastened,  is  placed  in  the  upper  part  of  the  vessel  G. 

The  balloon-shaped  glass 
B,  filled  with  crystals  of 
copper  sulphate,  closes  the 
cell.  The  cell  is  filled  to 
about  three-fourths  of  its 
capacity  with  a  solution 
of  1  part  crystallized  mag- 
nesium sulphate  in  7  parts 
of  water ;  and  the  balloon- 
shaped  flask  containing 
copper  sulphate  is  filled 
up  with  water,  closed  with 
a  stopper  fitted  with  the 
glass  tube  r,  and,  as  the 
FIG.  3.  cut  shows,  inverted  in 

the  cell. 

The  cells  used  by  the  author  have  the  following  dimen- 
sions :  — 


Jar      .... 

Flask.  .  .  . 
Zinc  cylinder  . 
Copper  cylinder 


21.5  cm.  high,  15  cm.  diameter. 
21.0    " 

10.0    "       "     12    "  " 

6.5    "      "       6    "          " 


The  strength  of  the  current  from  a  battery  of  Meidinger 
cells  was  determined  as  follows  : 

OH  Gas  per  Minute. 

2  Meidinger  elements  gave,  in  the  voltameter,  0.3  cc. 
4  Meidinger  elements  gave,  in  the  voltameter,  0.4  cc. 
6  Meidinger  elements  gave,  in  the  voltameter,  0.7  cc. 


DANIELL    CELL. 


11 


6  freshly  filled  Meidinger  cells  gave,  after  two  days'  use, 
1.5  cc. ;  after  eight  days,  1.9  ;  after  fourteen  days,  2.6  ;  after 
three  weeks,  3.1  cc.  oxyhydrogen  gas.  After  this  time,  the 
battery  remained  constant. 


DANIELL    CELL. 


In  a  jar  of  glass  (Fig.  4)  is  a  porous  clay  cup  T,  and  in 
this  a  cylinder  of  cast  zinc,  the  negative  pole  (Fig.  5).     The 


FIG.  4. 


FIG.  5. 


porous  cup  is  surrounded  by  a  cylinder  of  sheet-copper  K, 
the  positive  pole. 

The  cylinder  of  amalgamated  zinc  *  stands  in  dilute  sul- 

*  The  zinc  is  easily  amalgamated  by  plunging  it  into  mercury,  on  the 


12  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

phuric  acid  (1  :  20),  and  the  copper  cylinder  in  a  solution  of 
-copper  sulphate  ;  the  sulphuric  acid  may  be  replaced  by  a 
solution  of  zinc  sulphate. 

The  author  has  in  use  Daniell  cells  of  the  following 
dimensions :  — 

Jar  .     .     .     .     .  20.0  cm.  high,  10.4  cm.  diameter. 

Porous  cup    .     .  20.4    "       "        7.0    "  " 

Copper  cylinder,  17.0    "       "       8.5    "          " 

Zinc  cylinder      .  19.0    "       "        6.0    "  " 

[The  modification  of  the  Daniell  cell  known  as  the  gravity 
cell  is  the  form  commonly  in  use  for  telegraph  batteries  in 
this  country,  and  is  the  cheapest  and  most  convenient  cell 
for  constant  batteries  to  yield  currents  of  moderate  strength 
in  scientific  laboratories.  It  is  very  generally  thus  used. 
The  copper  is  placed  at  the  bottom  of  the  jar ;  an  insulated 
copper  wire  is  riveted  to  it,  long  enough  to  pass  up  through 
the  solutions  and  connect  with  a  binding  screw  on  the  zinc 
of  an  adjacent  cell,  or  with  the  wire  which  serves  to  conduct 
the  current  to  the  solution  for  electrolysis.  The  bottom  of 
the  jar,  about  the  copper,  is  filled  with  copper  sulphate ;  the 
zinc,  a  heavy  casting  with  large  surface,  is  suspended  a  few 
inches  below  the  top  ;  and  the  jar  is  filled  with  water  some- 
times acidulated  with  sulphuric  acid.  After  standing  a  few 
hours,  the  copper  sulphate  dissolves  ;  copper  is  precipitated, 
and  zinc  dissolved ;  and  the  jar,  in  its  normal  working  state, 
thus  contains  two  solutions ;  the  heavier,  of  copper  sulphate, 
below,  and  the  lighter,  of  zinc  sulphate,  above.  The  porous 
cup  of  the  Daniell  cell  is  thus  dispensed  with,  and  the  zinc 
does  not  require  amalgamation. 

surface  of  which  a  little  hydrochloric  acid  has  been  poured.  The  amalga- 
mated cylinder  is  then  placed  in  a  vessel  of  water  to  remove  the  hydrochloric 
acid,  and  allow  the  excess  of  mercury  to  drop  off. 


GRAVITY    CELL. 


The  cut  (Fig.  6)  shows  one  of  the  simplest  gravity  cells, 
having  the  zinc  in  the  so-called  "  crow-foot "  shape,  hanging 
directly  on  the  edge  of  the  jar,  and  furnished  with  a  binding- 


screw. 


FIG.  6. 


The  outfit  of  the  chemical  laboratory  of  the  Pennsylvania 
State  College,  while  under  the  translator's  charge,  was  found 
convenient,  and  sufficient  for  the  needs  of  an  ordinary  labora- 
tory for  instruction.  Some  twenty  "crow-foot"  gravity  cells 
were  kept  in  working  condition,  and  eight  Grove  cells  could 
be  set  up  if  needed  for  a  strong  current.  Four  sets  of  con- 
necting-wires were  run  from  the  battery-room  to  the  labora- 
tory desk  set  apart  for  electrolytic  work,  each  set  being  so 
arranged  with  binding-screws  as  to  be  quickly  connected  with 
any  desired  number  of  cells.  (See  under  "  Secondary  Bat- 
teries," p.  47.)  —  Trans.] 


14 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


GROVE    CELL. 

The  positive  pole  is  a  sheet  of  platinum  foil  of  the  form 
shown  in  Fig.  7 ;  this  is  placed  in  a  porous  cup  filled  with 
nitric  acid.  The  negative  pole  is  a  cylinder  of  amalgamated 
zinc  placed  in  a  glass  jar  containing  dilute  sulphuric  acid 
(1 :  20).  Fig.  8  shows  the  arrangement  of  the  cell. 


FIG.  7. 


FIG.  8. 


The  cells  used  by  the  author  have  the  following  dimen- 
sions :  — 


Jar  .     .     .     . 

Porous  cup  . 
Zinc  cylinder 
Platinum  foil 


13.5  cm.  high,  10.0  cm.  diameter. 


13.0  "  "  5.2  " 
14.0  "  "  8.5  " 
11.0  "  " 


BUNSEN    CELL. 


15 


BUNSEN    CELL. 

In  the  Bunsen  cell,  the  platinum  is  replaced  by  a  prism 
of  retort  carbon  (Fig.  9)  standing  in  a 
porous  cup  filled  with  nitric  acid.    The 
negative   electrode,   as    in    the    Grove 
cell,  is  a  cylinder  of  amal- 
gamated zinc  placed  in  a 
glass  jar  filled  with  dilute 
sulphuric    acid    (1  :  20). 
The    screw-clamp    shown 
in  Fig.  10  is  often  used  to 
fasten  a  metallic  connec- 
tion to  the  carbon  prism. 
It   has,  however,  the  dis- 
advantage that  the  clamp 
is  quickly  oxidized   by  the  decomposition  products  of  the 


FIG.  9. 


FIG.  10. 


FIG.  11. 


FIG.  12. 


16  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

nitric  acid,  and  the  contact  thus  broken.  It  is  better,  there- 
fore, to  insert  in  the  carbon  a  metallic  socket  (Fig.  11),  the 
stem  of  which  is  closely  covered  with  platinum  foil. 

Fig.  12  shows  the  Bunsen  cell  in  its  most  common  form. 

The  author  uses  cells  of  the  following  dimensions  :  — 

Jar 20.0  cm.  high,  11.5  cm.  diameter. 

Porous  cup    .     .  19.5    "       "        9.0    "  " 

Zinc  cylinder     .  20.0    "       "        9.0    "  " 

Carbon      .     .     .  21.0    "       "        5.0  cm.  wide,  2.4  cm.  thick. 

THERMO-ELECTRIC  PILES. 

The  thermo-electric  piles  in  use  are  Clamond's  and  Noe's. 

(Diamond's  pile  (shown  in  Figs.  13  and  14)  is  composed 
of  a  large  number  of  elements,  each  consisting  of  a  bar  of  an 
antimony  and  zinc  alloy,  and  a  strip  of  tinned  sheet-iron  ;  the 
iron  strips  are  fastened  to  the  bars  as  shown  in  Fig.  15,  thus 
serving  to  connect  the  elements.  Both  the  single  elements 
and  the  superimposed  rings  of  elements  are  separated  by 
layers  of  asbestus. 

The  poles  of  each  ring  of  elements  end  in  binding-screws. 
The  current  is  produced  by  heating  with  illuminating  gas 
which  burns  from  a  cylinder  of  clay  or  porcelain  perforated 
with  numerous  openings,  which  stands  in  the  middle  of  the 
pile  (Fig.  16,  one-third  natural  size).  This  tube-burner  is 
cemented  in  the  cylinder  with  a  mixture  of  powdered 
asbestus  and  water-glass,  and  can  be  replaced  in  case  of 
accidental  breakage.  To  keep  the  flow  of  gas  constant,  and 
prevent  excessive  heating  of  the  burner,  the  gas  is  first  passed 
through  a  regulator  filled  with  water  (r,  Fig.  13),  the  valve 
of  which  partly  closes  the  orifice  when  the  pressure  rises, 
and  opens  it  wider  when  the  pressure  falls.  The  water  in 


THERMO-ELECTRIC    PILES. 


17 


FIG.  14. 


18 


QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 


the  regulator  must  be  replaced  as  it  evaporates.  The  current 
attains  its  full  strength  when  the  gas  has  been  burning  about 
one  hour  ;  it  then  yields  400-450  cc.  oxy hydrogen  gas  per  hour. 


FIG.  15. 


FIG.  16. 


After  using  the  pile,  care  must  be  taken  not  to  cool  the 
tube-burner  too  quickly.  To  this  end,  the  cylinder  opening 
at  C  (Fig.  14)  is  first  closed  with  an  iron  plate  d ;  after  that 
the  cock  is  closed. 

The  elements  of  Noe's  thermopile  are  rods  of  an  alloy  con- 
taining 63  per  cent  antimony  and  37  per  cent  zinc,  about  7 
mm.  in  diameter  and  27  mm.  long  (Fig.  17),  to  each  of  which 
is  attached  a  smaller  pointed  iron  rod  (e)  to  conduct  the  heat 
to  it.  The  elements  are  arranged  in  a  circle,  on  a  ring  of 
ebonite,  with  the  iron  point  resting  on  a  plate  which  serves 
to  spread  the  flame  of  the  gas-burner  (Fig.  18).  The  con- 


THERMO-ELECTRIC    PILES. 


19 


nection  of  the  elements  by  German-silver  strips,  nn,  etc.,  is- 
shown  in  Fig.  19.  The  elements  are  soldered  to  copper 
plates  set  in  a  circle,  which  are  bent  in  spiral  form,  and 


FIG.  17. 


FIG.  18. 


FIG.  19. 


20 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


serve  to  support  the  elements,  and  also  to  cool  their  outer 
ends.  Fig.  20  gives  a  general  view  of  the  Noe  thermopile. 
If  it  is  only  moderately  heated,  the  air  cools  it  sufficiently ; 
if  more  strongly  heated,  it  must  be  placed  in  a  vessel  of 
water. 


FIG.  20. 

According  to  v.  Waltenhofen,  a  pile  of  128  elements  in 
4  groups  of  ,32  each  is  equal  in  electro-motive  force  to  about 
2  Daniell  cells. 

The  results  of  the  author's  investigations  are  not  favor- 
able to  the  use-  of  thermo-electric  piles  for  analytical  pur- 
poses ;  they  d<S«ot  give  a  current  strong  enough  for  most 
determinations  and  separations,  and  are  liable  to  fail  after 
continued  use  ;  and  then^can  be  repaired  only  with  difficulty, 
often  not  at  all. 


THERMO-ELECTRIC    PILES.  21 

[The  objections  made  by  the  author  in  the  preceding 
paragraph  are  so  well  taken  that  his  own  practice,  and  that 
of  other  workers  in  the  same  field,  is  to  use  other  sources  of 
current,  to  the  exclusion  of  thermopiles.  If,  however,  a 
thermopile  could  be  found  which  should  not  be  complicated, 
delicate,  high-priced,  liable  to  fail,  and  difficult  to  repair,  and 
which  should  yield  a  sufficient  current,  it  would  obviously  be 
an  ideal  source  of  electricity  for  a  small  isolated  laboratory, 
or,  indeed,  for  any  laboratory,  for  certain  purposes  at  least. 
Indeed,  a  thermopile  which  can  perform  the  work  of  the 
galvanic  battery  with  less  trouble,  attention,  and  repair,  holds 
the  same  advantage  as  a  source  of  current  for  analytical  pur- 
poses over  the  galvanic  battery  that  the  long-sought-for  ideal 
method  of  converting  the  potential  energy  of  coal  directly 
into  electricity,  instead  of  taking  the  long  and  wasteful  path 
through  furnace,  boiler,  steam-engine,  and  dynamo,  will  have, 
when  it  comes,  over  the  dynamo-plant,  now  the  accepted 
method  of  producing  electricity  on  the  large  scale.  Especially 
is  such  a  thermopile  suited  for  use  with  secondary  batteries ; 
the  thermopile  can  be  left  without  attention  for  any  time 
desired,  to  charge  the  secondary  battery,  which  shall  be 
directly  used  for  the  electrolytic  work  (see  "  Secondary  Bat- 
teries," p.  37). 

By  the  courtesy  of  the  inventor,  Dr.  Leonard  Paget,  of 
New  York,  the  translator  is  enabled  to  give  a  description  of 
a  thermopile  which  meets  these  conditions  in  a  remarkable 
degree.  It  can  be  constructed  by  any  chemist  or  electrician. 

It  consists  of  thin  annular  disks  of  copper  and  German 
silver  buckled,  or  dished  (Fig.  21),  and  placed  alternately,  one 
above  another,  upon  the  gas-tube  g  (Fig.  22),  so  that  adjacent 
disks  are  held  in  contact  by  their  own  elasticity.*  The  whole 

*  The  gas-tube  #  in   this  small  pile  is  conveniently  made  of  asbestos 
sheet,  shaped  into  tube  form  about  a  stick  of  the  desired  diameter. 


"22  QUANTITATIVE  ANALYSTS   BY    ELECTROLYSIS. 


FIG.  21. 


FIG.  22. 


FIG.  23. 


THERMO-ELECTRIC   PILES.  23 

system  is  held  between  annular  iron  plates  pp  (Fig.  23)  with 
asbestos  washers  ww,  by  three  long  bolts  &  ;  these  bolts  are 
prolonged  to  serve  as  legs.  Heat  is  supplied  by  a  Bunsen 
burner.  The  disks  are  some  3  or  4  inches  in  diameter,  with 
an  opening  1  inch  in  diameter. 

This  thermopile  can  be  taken  apart  at  any  time  by  re- 
moving three  nuts,  and  the  contact  edges  of  the  disks  bright- 
ened by  sand-papering.  This  will  be  found  desirable  after 
several  weeks'  use.  and  can  be  done  in  an  hour  by  unskilled 
labor. 

A  larger  form,  used  with  great  success  in  copper  deter- 
minations at  the  Chicago  Copper  Refining  Works,  consisted 
of  disks  8  or  more  inches  in  diameter,  with  a  3-inch  opening, 
and  was  heated  by  charcoal,  the  gas-tube  g  being  furnished 
with  a  simple  grate  and  extended  upward  to  produce  a 
draught.  This  latter  form  has  been  used  in  the  works  re- 
ferred to,  in  preference  to  an  available  side  current  from  a 
dynamo. 

Fig.  24  sho\vs  a  still  larger  form,  heated  by  coke  on  the 
base-burning  principle,  which  was  constructed  and  used  in 
1892,  by  permission  of  Dr.  Paget. 

The  fire-box  is  1  foot  in  diameter.  The  annular  copper 
plates  c  are  extended  to  form  cooling  plates;  all  air  used  in 
combustion  being  drawn  over  them,  as  indicated  by  arrows. 
The  cylinder  WHS  2  ft.  high,  with  8  pairs  of  elements  to  the 
inch,  and  the  output  in  actual  work  was  11  volts  and  72  am- 
peres. 

Various  modifications  will  suggest  themselves  under 
special  conditions,  e.g.,  the  adaptation  of  the  size  of  the  disks 
to  the  use  of  one  of  the  more  powerful  oil  lamps  as  a  source  of 
heat. 

Dr.  Paget  uses  disks  ^  of  an  inch  in  thickness.  He  finds 
that  about  35  pairs  of  the  smaller  size  described  give  an  e.rn.f. 


24  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS 

of  1  volt,  so  that  to  charge  two  secondary  cells  would  require 
about  200  pairs. 

The  apparatus  is  patented,  but  Dr.  Paget  freely  gives  the 
use  of  it  to  his  fellow  scientific  workers,  and  will  be  glad  to 


FIG.  24. 


answer  any  inquiries  as  to  details.     Such  inquiries  may  be 
made  through  the  translator.  —  Trans.'] 


ELECTRICAL  MACHINES. 

For  electro-chemical    processes,  uniform    continuous  cur- 
rents of  great  quantity  (low  intensity)  are  required. 


ELECTRICAL    MACHINES.  25 

The  author  used  for  electrolytic  analysis,  during  the  years 
1881-5,  a  magneto-electric  machine  made  by  Siemens  & 
Halske  of  Berlin. 

A  pulley  was  fixed  on  the  axis  of  this  machine,  and  con- 
nected to  a  second  pulley  on  a  counter-shaft.  The  counter- 
shaft carried  a  cone  pulley  with  5  steps  of  30,  25,  20,  15,  and 
10  cc.  diameter,  corresponding  to  a  similar  cone  pulley  on  a 
second  counter-shaft.  This  second  counter-shaft  was  provided 
with  fast  and  loo^e  pulleys,  and  was  directly  connected  with 
the  source  of  power.  The  change  of  connection  of  the  cone 
pulleys,  therefore,  changed  the  velocity  of  revolution  of  the 
magneto-electric  machine. 

The  observed  velocities  of  the  machine,  with  this  arrange- 
ment, were  700,  500,  300,  200,  and  100  revolutions  per 
minute. 

To  control  still  more  closely  the  strength  of  the  current, 
a  regulator  was  inserted  provided  with  resistance-spirals  and  6 
contact?,  giving  resistances  of  0.01,  0.02,  0.06,  0.6,  1.45,  and 
3  ohms;  thus  the  machine  was  made  available  for  all  deter- 
minations and  separations. 

This  arrangement  is  adapted,  as  will  be  seen,  to  carry 
•on  simultaneously  only  similar  determinations;  it  is  not 
possible,  e.g.,  to  determine  together  iron  and  antimony  or 
•copper. 

The  firm  of  Siemens  &  Halske  has  constructed,  for  the 
laboratory  of  the  author,  an  apparatus  which  allows  a  large 
number  of  the  most  unlike  determinations  to  be  carried  on 
together  without  interference.  The  action  of  the  apparatus 
depends  essentially  on  an  arrangement  by  which  the  full 
current  of  the  machine  is  sent  through  an  artificial  resistance 
with  many  subdivisions,  and  the  tension  of  these  subdivisions 
is  kept  constant ;  that  is,  each  subdivision  has  a  constant 
known  tension,  which  remains  unchanged,  if  a  side  current 


20  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

of  comparatively  little  strength  is  taken  to  carry  on  a  deter- 
mination. 

Before  describing  the  details  of  the  apparatus,  it  must  be 
premised  that  the  current  is  produced  by  a  dynamo  machine 
of  the  form  shown  in  Fig.  25.  This  machine,  at  1,000  revo- 


FIG.  25. 

lutions  per  minute,  furnishes  a  current  of  60  amperes  with  a 
tension  of  10  volts  ;  it  requires  to  run  it  something  more 
than  one-horse  power. 

Siemens  &  Halske  describe  the  action  of  the  dynamo 
as  follows :  A  current  is  produced  in  a  closed  electric  con- 
ductor when  a  portion  of  it  is  passed  between  opposite 
poles,  which  may  be  developed  in  fixed  or  in  moving  masses 


ELECTRICAL    MACHINES. 


South 


North 


of  iron.  The  direction  of  this  current  depends  on  the- 
position  of  the  magnetic  poles  with  reference  to  the  direction 
of  the  motion.  The  con- 
ductor, by  the  motion  of 
which  the  electric  current 
is  produced,  is  insulated 
copper  wire  wound  in  sev- 
eral divisions  of  many  turns 
each  about  an  iron  core,  so 
as  to  cover  it  completely, 
even  on  the  faces.  This 
core  is  a  hollow  cylinder  of 
soft  iron  wires  or  plates, 
which  revolves  on  an  axis 
passing  longitudinally 
through  it  (Fig.  26,  nn', 
ss').  Partly  surrounding 
this  hollow  cylinder  on 
either  side,  and  conforming 
to  it  in  shape,  are  iron  bars, 
NN',  SS',  the  straight  pro- 
jecting portions  of  which 
are  wound  with  insulated  FlG 

copper  wire,  and  connected 

by  the  bars  m  and  O,  thus  forming  horse-shoe  electro-magnets,. 
NwS  and  N'OS',  with  their  similar  poles  opposite  to  each 
other. 

By  the  action  of  the  electro-magnets,  powerful  opposite 
magnetic  poles  are  formed  in  the  iron  bars  to  the  right  and 
left  of  the  rotating  wire  covering  of  the  core. 

The  iron  core  becomes,  by  induction,  a  transverse  magnet 
always  opposing  its  poles  to  those  of  the  outer  electro- 
magnets. The  intermediate  spaces,  in  which  revolves  the 


•28  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS 

wire  cylinder  covering  the  core,  become  magnetic  fields  of 
great  intensity.  Every  revolution  of  the  wire  cylinder  pro- 
duces in  each  turn  of  the  wire,  as  it  passes  through  the  two 
magnetic  fields,  two  currents  in  opposite  directions.  From 
the  combined  action  of  these  single  alternating  currents,  by 
means  of  a  commutator  which  is  connected  in  a  peculiar 
manner  with  the  single  coils,  a  continuous  current  (constant 
in  one  direction)  is  produced.  The  commutator  C  (Fig.  25) 
consist^  of  a  number  of  insulated  copper  plates,  which,  taken 
togelt-he'y,  form  a  cylinder  surrounding  the  axis  of  the  iron 
core^and  revolving  with  it.  Brushes  of  copper  wire  transmit 
the  current  from  the  commutator  to  the  wire  that  forms  the 
circuit. 

According  to  the  fundamental  principle  of  the  dynamo, 
the  electric  current  which  is  produced  is  itself  utilized  for 
strengthening  the  magnetism  which  is  necessary  to  the 
machine,  the  weak  magnetism  remaining  in  the  iron  being 
sufficient  to  begin  the  action  when  the  machine  is  started. 
The  current,  to  this  end,  traverses  the  wires  which  are 
wound  about  the  electro-magnets,  as  well  as  the  external 
circuit,  in  which  it  is  utilized. 

As  already  stated,  the  machine  described  has  a  tension  of 
10  volts  with  1,000  revolutions.  The  tension,  while  the 
machine  is  in  use,  is  measured  by  a  galvanometer  or  other 
instrument  which  shows  the  tension  directly.  In  Fig.  27,  the 
tension  indicator  marked  G  is  connected  with  both  ends  of 
the  brass  resistance  MMr 

Siemens  &  Halske  describe  the  tension  indicator  as 
follows :  It  consists  of  an  electro-magnet,  beside  one  pole  of 
which  stands  on  edge  a  piece  of  iron  which  has  the  same 
polarity,  and  is  therefore  repelled  by  it  in  proportion  to  the 
strength  of  the  magnetism,  and  so  of  the  electric  current 
which  passes  around  the  instrument.  The  extent  of  the 


ELECTRICAL    MACHINES. 


repulsion  is  measured  on  a  scale  on  which  plays  an  index 
attached  to  the  piece  of  iron  which  is  repelled.  The  indica- 
tions of  the  instrument  are  not  entirely  independent  of  the 


residual  magnetism ;  the  direction  of  the  current  in  the 
instrument  must  therefore  be  always  the  same.  This  result 
is  accomplished  by  a  small  adjustable  permanent  magnet  in 


30  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

front  of  the  lower  pole  of  the  electro-magnet ;  this  shows  the 
direction  of  the  current  in  the  instrument,  and  stops  the 
index  if  the  current  is  in  the  wrong  direction.  (See  Z, 
Fig.  27.)  If  this  occurs,  the  wires  leading  the  current  to 
the  instrument  must  be  interchanged. 

The  instrument  is  supplied  with  a  brass  ring,  which, 
before  the  current  passes,  is  placed  on  the  round  weight  of 
the  index,  and  must  then  turn  the  index  to  the  zero  point. 
If  this  is  not  the  case,  the  instrument  is  not  plumb.  When 
the  instrument  is  in  use,  the  ring  is  removed. 

As  already  stated,  the  laboratory  apparatus  constructed 
by  Siemens  &  Halske  is  capable  of  carrying  on,  at  the  same 
time,  a  large  number  of  electrolytic  determinations  on  the 
small  scale,  requiring  currents  differing  in  strength  and  ten- 
sion, so  that  each  determination  is  independent  of  the  rest. 
According  to  the  description  of  Siemens  &  Halske,  this 
result  is  obtained  essentially  by  passing  far  the  greater  part 
of  the  current  through  a  brass  wire-gauze  resistance,  the 
individual  determinations  being  made  by  small  branch  cur- 
rents which  may  be  independently  varied  in  intensity  by 
attaching  their  conductors  to  different  portions  of  the  wire- 
gauze  resistance. 

The  dynamo  machine  is  connected  by  short  heavy  con- 
ductors to  the  ends  M  M1  of  the  zigzag  brass  wire-gauze 
resistance.  These  ends  of  the  resistance  are  also  connected, 
by  smaller  wires,  with  the  instrument  which  shows  directly 
the  tension  at  the  resistance.  Care  must  be  taken  that  this 
instrument  always  shows  the  same  tension,  i.e.,  that  the 
velocity  of  the  machine  is  uniform.*  If  the  tension  at  the 
ends  of  the  resistance  is  6  volts,  and  the  resistance  is  made 


*  In  the  laboratory  of  the  author,  the  machine,  to  accomplish  this,  is 
driven  by  a  small  gas  motor. 


ELECTRICAL    MACHINES.  31 

up  of  24  equal  parts,  the  ends  of  which  are  connected  with 
binding-screws,  the  difference  in  tension  between  any  two 
adjacent  binding-screws  is  ^  =  J  volt.  If  the  tension  at 
the  first  screw  is  0,  the  tensions  at  the  following  screws  are 
1»  f »  f  i  1?  f  >  etc.,  volts ;  that  is,  the  whole  interval  of  6  volts 
is  divided  into  portions  of  J  volt  each. 

If,  now,  a  current,  small  in  proportion  to  the  current 
passing  through  the  resistance,  is  taken  out  between  any  two 
binding-screws  for  an  electrolytic  determination,  the  tension 
between  the  screws  is  not  materially  changed ;  the  wires 
carrying  this  current  can  be  connected  with  any  binding 
screws  without  any  change  in  the  main  current ;  moreover, 
the  introduction  of  a  number  of  such  currents  does  not 
materially  change  the  tension,  and  the  tension  for  any  given 
determination  can  be  varied  at  will  without  affecting  the 
others, 

In  the  apparatus  used  by  the  author,  Fig.  27  (one- 
twentieth  natural  size),  the  brass  wire  gauze  resistance  is 
divided  into  20  equal  parts  marked  1,  2,  3,  etc.  As  already 
stated,  the  machine,,  at  IjOOO  revolutions,  has  a  current- 
strength  of  60  ampdres  and  a  tension  of  10  volts.  Of  the  60 
amperes,  40  are  conducted  through  the  resistance,  so  that  20 
remain  for  electrolytic  determinations. 

The  difference  of  tension  between  two  adjacent  binding- 
screws  is  J§  —  i  volt.  The  tension,  that  is,  at  the  screw 
marked  19,  is  £  volt,  at  18  =  1,  at  17  =  1J,  at  16  =  2,  at 
0  =  10  volts. 

The  current  from  the  machine  enters  by  a  heavy  copper 
conductor  at  the  screw  marked  0,  and  passes  out  at  that 
marked  20. 

On  the  board  BBBB  are  fastened  6  T-shaped  galvanized- 
iron  strips,  Sx,  S2,  S3,  S4,  S6,  S6,  six  resistances  of  0.1  ohm 
each,  Wu  W2,  W3,  W4,  W5,  W6  (to  allow  the  strength  of 


82  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

current  in  single  experiments  to  be  measured),  and  the  brass< 
strip  M2.  8j  is  connected  b}^  a  wire  with  Wv  S2  with  W2,  S3 
with  W8,  S4  with  W4,  S5  with  W6,  and  S6  with  W6.  The  iron 
strips  may  be  connected  with  the  binding-screws  1,  2,  3,  etc.,, 
by  means  of  wires  and  the  brass  screws  Kv  K2,  etc.  If  the 
apparatus  is  used  as  shown  in  the  cut,  and  1,  2,  or  3  is 
connected  with  Sx,  4,  5,  or  6  with  S2,  7,  8,  or  9  with  S8,  10, 
11,  or  12  with  S4,  13,  14,  15,  or  16  with  S6,  and  one  of  the 
others  with  S6,  the  strongest  current  is  at  Wv  and  the  weakest 
at  W6.  Any  strip  may,  of  course,  be  connected  with  any 
binding-screw. 

In  performing  electrolysis,  the  solutions  to  be  acted  on 
are  placed  in  connection  with  a  negative  pole  nv  n2,  or 
w3,  etc.  (on  the  resistances  Wv  W2,  or  W3,  etc.),  and  a 
positive  pole  pv  p%,  or  p%,  etc.,  on  the  brass  strip  M2,  the 
connections  being  made  according  to  the  strength  of  current 
desired. 

Moreover,  as  shown  by  the  examples  given  later,  several 
determinations  requiring  the  same  strength  of  current  may 
be  connected  with  any  pair  of  poles,  n^  and  pv  n%  and  jt?2,  etc.. 
In  order  to  connect  more  conveniently  with  the  platinum 
dishes  containing  the  solutions  for  electrolysis,  n^  and  pv  for 
instance,  may  be  connected  with  a  brass  strip  Z  (the  con- 
nection with  W]_  only  is  shown  in  the  cut),  to  which  are 
attached  a  number  of  binding-screws,  zv  «2,  etc. 

The  tension  and  the  strength  of  the  current  may  be 
measured  at  each  dish.  For  example,  if  the  tension  at  the 
dish  connected  with  W2  is  to  be  measured,  the  plugs  from 
the  galvanometer  are  inserted  at  £>2  and  c2 ;  if  they  are 
inserted  at  a2  and  Z>2,  the  tension  in  the  resistance  is  meas- 
ured, which,  multiplied  by  10,  gives,  in  amperes,  the  strength 
of  the  current  acting  on  the  solution  connected  with  W2. 

In  order  to  test  the  working  of  the  apparatus,  the  tension 


ELECTRICAL    MACHINES. 


at  the  divisions  of  the  wire-gauze  resistance  was  directly 
measured  by  a  torsion  galvanometer,  with  the  following 
results : 


Resistance  marked 

Connected  with  Binding 
Screw,  marked 

Tension  in  Volts. 

W, 

1 

10.300 

wi 

2 

9.900 

Wl 

3 

9.400 

W2 

4 

8.950 

W2 

5 

8.300 

W2 

6 

7.750 

W3 

7 

7.200 

W3 

8 

6.650 

W3 

9 

5.950 

W4 

10 

5.500 

W4 

11 

5.050 

W4 

12 

4.500 

W5 

13 

4.000 

W5 

14 

3.450 

W5 

15 

2.850 

W5 

16 

2.300 

W6 

17 

1.700 

W6 

18 

1.100 

W6 

19 

0.560 

W6 

20 

0.007 

For  the  measurement  of  the  strength  of  the  current  at 
the  screws  1  to  20,  a  cell  was  iised  which  had  a  copper  elec- 


34  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

trode,*  and  contained  150  cc.  of  a  15  per  cent  solution  of 
copper  sulphate  ,  this  cell  was  connected  to  the  resistance  W6 
(binding-screws,  ne  and  pQ).  The  screws  1  to  20  were  then 
successively  connected  with  the  bar  S6,  and  the  deviation  of 
the  galvanometer  read,  the  plugs  connecting  it  being  placed 
in  aQ  and  56.  After  this  reading,  the  tension  in  the  cell  was 
read,  for  each  screw  connection,  by  placing  the  plugs  in  bQ 
and  CQ. 

In  order  to  control  the  rate  of  the  machine  during  the 
experiment,  pl  and  nl  on  the  resistance  W1  were  connected 
through  a  rheostat ;  and  the  tension  at  the  binding-screw  1 
(connected  with  Sj)  was  determined  by  a  second  torsion 
galvanometer,  the  plugs  from  which  were  inserted  at  b1 
and  cr 

The  results  of  these  experiments  are  given  in  the  following 
table  in  the  columns  included  under  I. 

A  second  series  of  experiments  was  conducted  to  deter- 
mine the  strength  of  the  current  by  the  quantity  of  copper 
precipitated. 

Six  platinum  dishes,  as  nearly  alike  as  possible,  were 
filled* with  150  cc.  each  of  a  15  per  cent  solution  of  copper 
sulphate,  supplied  with  copper  eiectrodes  (see  note  below), 
and  different  quantities  of  copper  precipitated  in  the  same 
time.  These  experiments  were  conducted  in  three  series,  as 
follows :  — 

Series  1.  I.,  IV.,  VIII,  XII,  XVI,  XIX. 
Series  2.  II,  V,  IX,  XIII,  XVII,  XX. 
Series  3.  Ill,  VI,  X,  XI,  XIV,  XVIII. 


*  The  cell  consisted  of  a  platinum  dish,  and  the  positive  electrode  was  a 
round  piece  of  sheet-copper  (of  the  form  of  the  platinum  electrode  shown  in 
Fig.  37),  6  cm.  in  diameter  and  2  mm.  thick.  The  electrodes  were  2.5  cm. 
apart. 


ELECTRICAL    MACHINES. 


35 


Of  the  columns  included  under  II.,  A  gives  the  strength 
of  the  current  as  determined  from  the  precipitated  copper ; 
B,  the  results,  in  a  few  cases,  of  the  measurement  of  the 
strength  of  current  by  a  torsion  galvanometer  ;  and  C, 
the  tension  measured  at  the  same  time  with  the  torsion 
galvanometer. 


Binding 
Screw. 

I. 

II. 

Amperes. 

Yolts>         Volts, 

ExPeri-     Machine, 
ment. 

A, 

Amperes. 

B,  Am- 
peres. 

c, 

Volts. 

I. 

II. 

18.018 
15.352 

7.900 
7.400 

10.90 
9.90 

15.97 
14.04 

- 

9.200 
9.000 

III. 
IV. 
V. 

13.231 
11.615 
10.302 

7.100 
6.650 
6.350 

10.10 
10.10 
10.30 

10.86 
8.87 
8.00 

10.800 

7.900 
7.400 
7.100 

VI. 

9.595 

6.010 

10.40 

6.04 

- 

5.500 

VII. 

8.383 

5.710 

10.50 

- 

- 

- 

VIII. 

6.565 

5.300 

10.60 

4.97 

- 

5.000 

IX. 

5.757 

5.100 

10.60 

4.21 

3.800 

4.500 

X. 

4.747 

4.700 

11.10 

4.03 

- 

3.800 

XI. 

4.040 

4.250 

10.90 

3.75 

3.700 

3.100 

XII. 

3.838 

3.800 

11.00 

3.54 

- 

2.900 

XIII. 

3.535 

3.400 

10.90 

3.47 

"  -„ 

2.500 

XIV. 

3.030 

2.850 

10.90 

8.09 

2.700 

.2.800_ 

XV. 

2.520 

2.400 

11.05 

- 

- 

- 

XVI. 

2.120 

1.900 

11.00 

1.85 

- 

1.200 

XVII. 

1.560 

1.500 

11.00 

1.35 

- 

1.050 

XVIII. 

0.759 

0.890 

10.90 

0.76 

0.605 

0.600 

XIX. 

0.396 

0.290 

11.00 

0.54 

- 

0.360 

XX. 

0.000 

0.007 

11.10 

0.00 

- 

0.007 

36 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


The  following  sixteen  experiments  were  made  simulta- 
neously under  the  same  conditions  as  before.  The  numbers- 
in  column  A  express  the  quantities  of  copper  precipitated  in 
6.5  minutes ;  those  under  B,  the  tensions  measured  with  the 
torsion  galvanometer. 


A,  Copper. 

B,  Volts. 

(     0.7616  gm., 

Binding  screw  I.  to  W1     .     . 

\     0.7415    " 

7.10 

1     0.8286    "       J 

f    0.6021    "        1 

Screw  IV.  to  W2     .... 

\     0.5716    " 

5.30 

[     0.4788    "       J 

0.4155    "        1 

Screw  VIII.  to  W8  .     .     .     . 

0.3510    " 

3.30 

0.3535    "       J 

'    C.2648    "       1 

Screw  XII.  to  W4   .... 

•     0.2963    " 

1.80 

0.2652    "        J 

Screw  XVI.  to  W6  .     .     .     . 

j     0.1435    "       | 
I     0.1470    "        J 

0.90 

Screw  XIX.  to  W6  .     .     .     . 

i 

(     0.0363    "        | 
1    0.0260    "       J 

0.23 

In  order  to  reach  a  conclusion  as  to  the  value  of  the 
apparatus  for  the  purposes  of  quantitative  analysis,  twelve 
determinations  were  carried  on  simultaneously,  at  the  author's 
request,  by  Dr.  Robert  Ludwig,  formerly  assistant  in  the  In- 
organic Laboratory.  The  solutions  used  for  these  experi- 
ments were  of  iron,  cobalt,  tin,  antimony,  and  copper,  metals 


SECONDARY    BATTERIES. 


37 


which,  as  will  be  shown  later,  require  currents  of  widely 
different  strengths  for  their  separation.  The  results  of  one 
series  of  these  experiments  are  subjoined. 


Taken. 

Found. 

I. 

0.3546  gm.  Fe2O3 

0.2479  gm.  Fe  =    0.3541  gm.  Fe2O3 

II. 

0.3836    "    Fe203 

0.2691    "    Fe  =    0.3844    "    Fe2O3 

III. 

0.2624    "    Co 

0.2619    "    Co 

IV. 

0.2234    "    Co 

0.2231    "    Co 

Y. 

0.1145    "    Sn 

0.1142    "    Sn 

VI. 

0.2290    "    Sa 

0.2290    "    Sn 

VII. 

0.2025    "    Sb2S3 

0.1444    "    Sb  =    0.2019    "    Sb2S3 

VIII. 

0.1890    "    Sb2S3 

0.1348    "    Sb  =    0.1885    "    Sb2S3 

IX. 

0.1670    "    Sb2S3 

0.1189    "    Sb  =  .0.1663    "    Sb2S3 

X. 

0.8374    "    CuSO4 

0.2133    "    Cu  =  25.47  %  Cu 

XI. 

0.8768    "    CuSO4 

0.2225    "    Cu  =  25.31  %  Cu 

XII. 

0.7905    "    CuSO4 

i  0.1991    "    Cu  =  25.29  %  Cu 

i            Calculated  25.39  %  Cu 

[SECONDARY  BATTERIES. 

The  secondary  battery  ("accumulator,"  or,  more  com- 
monly, "  storage  battery  ")  was  already  beginning  to  attract 
general  attention  when  the  earlier  edition  of  this  work  was 
issued.  To  whatever  degree  it  has  succeeded  or  failed  in 
doing  the  varied  work  that  has  been  put  upon  it,  it  is  unques- 
tionably the  most  convenient  and  desirable  proximate  source 
of  the  electric  current  for  analytical  purposes. 

The  name  "  Secondary  Battery  "  accurately  describes  it, 
and  is  therefore  decidedly  preferable  to  either  "  Accumulator" 
or  "  Storage  Battery."  There  is  no  accumulation  and  no 
storage  of  electricity  ;  while  the  chemical  reaction  as  a  result 


88  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

of  which  the  battery  gives  out  an  electric  current  is  a  second- 
ary one;  that  is,  it  must  be  preceded  by  a  reverse  chemical 
action,  produced  by  an  electric  current  from  some  primary 
source. 

In  the  simplest  form  of  secondary  battery,  introduced  by 
Plante,  two  or  more  lead  plates,  alternately  positive  and  nega- 
tive, are  immersed  in  dilute  sulphuric  acid,  and  a  current  from 
a  primary  battery  is  passed  through  these  electrodes,  at  first 
alternately  in  opposite  directions,  until  the  cell  is  "  formed  " 
— that  is,  until  the  surface  of  the  lead  has  become  porousr 
g-iving  a  large  surface  for  chemical  action.  When  "  formed," 
the  charging  current  is  always  passed  in  one  direction.  To- 
expedite  and  intensify  the  "  forming,"  very  many  modifica- 
tions have  been  made  which  may  be  grouped  under  three 
heads  :  (1)  extending  the  surface  of  the  lead  plates,  (2)  pro- 
ducing oxidation  by  chemical  means,  (3)  applying  mechani- 
cally an  oxide,  or  mixture  of  oxides,  of  lead. 

The  reaction  in  the  secondary  battery  is  essentially  the 
decomposition  of  water.  When  the  electric  current  is  passed 
between  platinum  electrodes  through  acidulated  water,  as  in 
the  voltameter  (p.  58,  Fig.  33),  the  water,  as  is  well  known,  is- 
decomposed  into  oxygen  and  hydrogen.  If  now  the  enor- 
mously extended  lead  electrodes  are  substituted  for  the  plati- 
num electrodes,  the  oxygen  converts  the  lead  or  lead  monoxide 
of  the  positive  electrode  into  lead  peroxide,  while  the  hydro- 
gen reduces  any  lead  oxide  on  the  negative  electrode  to 
metallic  lead,  and  becomes  occluded  on  the  surface  of  tin's 
lend.  (Accordingly,  the  abundant  disengagement  of  gas- 
bubbles  marks  the  completion  of  the  charging  of  a  secondary 
battery.)  The  battery,  thus  charged,  is  chemically  in  the  con- 
dition of  a  primary  battery  —  that  is,  there  is  little  or  no  chem- 
ical action  until  the  opposite  electrodes  are  put  in  metallic 


SECONDARY   BATTERIES. 


39 


connection,  when  chemical  action  begins,  with  the  production 
of  an  electric  current. 

In  the  discharge,  substantially  the  reverse  reaction  takes 
place.  The  lead  peroxide  of  the  positive  electrode  is  reduced 
by  the  occluded  hydrogen  and  by  the  hydrogen  of  newly  de- 
composed water,  the  oxygen  of  which  brings  more  or  less  com- 
pletely to  the  state  of  monoxide  the  surface  of  the  negative 
electrode. 

Secondary  reactions,  between  lead  or  its  oxides  and  the  acid 
used,  occur  to  a  very  considerable  extent,  so  that  the  whole 
subject  of  the  reactions  in  the  secondary  battery  is  complex  ; 
the  outline  above  gives,  however,  the  fundamental  reaction, 
and  is  sufficient  for  our  purpose. 

Many  forms  have  been  given  the  secondary  battery,  and, 
indeed,  some  successful  batteries 
use  other  metals  than  lead.  Any 
of  the  batteries  to  be  found  in  the 
market  will  give  good  results.  Fig. 
28  is  a  cut  of  a  form  that  has  been 
very  generally  adopted  for  use  with 
the  phonograph.  It  shows  the  gen- 
eral appearance  both  of  the  com- 
plete cell  and  of  the  assemblage 
of  alternate  positive  and  negative 
electrodes  as  immersed  in  the 
acid. 

Moreover,  a  secondary  battery 
that  will  give  entirely  satisfactory 
results  for  analytical  purposes  can 
readily  be  made  in  any  laboratory, 

and  formed  by  the  current,  substantially  on  the  Plante  system. 
In  undertaking  this,  it  is  well  to  remember  that  complete 
isolation  of  the  electrodes  from  each  other  is  essential,  and 


FIG.  28. 


40  QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

that   extended    surface   is   favorable   to  rapid    and    compete 
"  forming." 

For  a  full  and  recent  com  pen  d  of  facts  in  regard  to  the 
secondary  battery  see  k<  The  Voltaic  Cell."  by  Park  Benjamin, 
pp.  415-508.  —  Trans.'] 

THE  USB  OF  ACCUMULATORS  AND   THEIR  ADVANTAGES 
OVER   DYNAMOS   AND   GALVANIC   BATTERIES. 

The  apparatus  described  on  pp.  25-32  for  carrying  on 
simultaneously  a  large  number  of  analyses  by  a  dynamo  cur- 
rent, which  was  used  for  years  only  in  the  Aachen  laboratory, 
has  the  disadvantage  that  at  night,  and  on  days  when  the 
stenm  engine  is  not  in  use,  electrolytic  work  must  cease.  This 
fact  led  the  author  to  experiment  with  accumulators,  using 
the  apparatus  constructed  by  Professors  Farbaky  and 
Scheneck,*  of  Schemuitz,  Hungary.  These  gentlemen  had 
the  kindness  to  place  two  accumulators  at  the  author's  service 
for  testing,  and  he  made  his  first  experiments  in  1888,  work- 
ing with  R.  Schelle,  at  that  time  professor  in  the  Schemnitz 
Royal  School  of  Mines.  These  accumulators  have  6  negative 
and  5  positive  lead-plate  electrodes,  each  6  mm.  thick.  The 
weight  of  the  electrodes  is  15.5  kg.,  the  volume  of  the  33$ 
sulphuric  acid  3.5  1.,  the  total  weight  of  each,  cell  35  kg. 
The  active  surface  of  the  electrodes  is  3133  sq.  cm.,  so  that 
the  internal  resistance  is  very  low,  measuring  between  0.0166 
and  0.0 17  ohm.  The  accumulators  can  be  charged  at  a  20 
to  25  ampere  rate,  and  yield  in  discharge  at  23,  30,  40  and  60 
ampere  rates  respectively  150,  148, 140  and  125  ampere-hours, 
with  a  fall  of  riot  over  10%  in  the  voltage.  If  the  discharge 

*  Cf.  Ueber  die  elektrische  Akkumulatoren  voii  Farbaky  und  Sche- 
neck  (Dingier,  Polyt.  Jour.  257,  357);  also  Bericht liber  die  Akkumulaloren 
vou  Farbaky  und  Scbeneck  von  A.  v.  Waltenbofeu,  Zeit.  f.  Elektro- 
technik,  1886. 


SECONDARY    BATTERIES.  41 

is  lighter,  and  the  fall  in  electro-motive  force  less  than  for 
lighting  purposes,  as  in  electrolytic  analyses,  an  accumulator 
may  yield  over  250  ampere-hours. 

Two  such  accumulators  were  fully  charged,  until  OH  gas 
was  obviously  disengaged,  by  a  current  of  20  to  25  amperes 
from  the  dynamo,  through  the  brass-wire-gauze  resistance, 
Fig.  27.  The  current  was  measured  by  a  Kohlrausch  galva- 
nometer, made  by  Hartmann  &  Braun,  Bockenheim,  Frank- 
furt a.  M.,  the  scale  of  which  read  from  0  to  60  amperes.  A 
second  Kohlrausch  amperemeter,  divided  from  0  to  15  am- 
peres, was  used  to  measure  the  current  taken  from  the  accu- 
mulators for  the  analyses. 

A  Siemens  torsion  galvanometer  showed  a  tension,  for  each 
charged  accumulator,  of  2.05  volts;  when  the  two  accumula- 
tors were  connected  for  tension,  they  yielded  40-50  cc.  oxy- 
hydrogen  gas  (see  p.  57)  per  minute. 

By  the  use  of  these  two  accumulators,  four  to  eight  analyses 
were  carried  on  simultaneously,  and  the  accumulators  kept  in 
constant  use  day  and  night,  except  for  the  short  intervals 
needed  to  change  the  solutions  for  analysis. 

The  results  of  analyses  extending  over  a  period  of  six  days 
are  subjoined. 

FIRST  DAY. 

Tension  2.55  volts.    (Voltameter  —  48  cc.  OH  gas.) 

Determination   of  Copper  from   Nitric-acid   Solution. 

Taken  CuSO4,5H2O.  Found  Cn. 

4.0140  gm.  1.0170    gm.  =  25.33$ 

4.1376    ^  1.0480     "     =  25.33 

2.2340    «  0.5661     "     =  25.34 

2.3575    «  0.5978     "     =  25.35 


42  QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS 

Tin  from  the   Acid   Ammonium   Double   Oxalate.* 
Taken  Si]Cl42NH4Cl. 


l.S450gm. 
2.0210    " 


Found  Sn. 
0.5964  gra.  =  32.33 
0.6548      ><    =  32.39 


Antimony  from  Solution  in  Sodium  Sulphide. f 

Taken  Sb2S3.  Found  Sb. 

n 

0.2404  gm.  0.1720   gm.  =  71.50 


0.2551    " 


0.1827 


=  n.eo 


SECOND  DAY. 


Tension  1.95  volts.     (Voltameter  =  42  cc.  OH  gas.) 
2.0490  gm.  NiSO4  +(NH4)2SO4,6H3O       gave    0.3053  gm.  Ni  =  14.90# 


( 
( 

3.0180    " 
3.3400    "     CoSO4-[-K2SO4,6H2O 

"      0.3000 
"      0.3440 

"  =14.91 

( 

3.1200    " 

"      0.3120 

"      "  =1471 

] 

89?0    "    FeSO4  +(NH4)2SO4,6H2O 

"      0.2697 

"     Fe  =  14.25| 

( 

3.1210    " 

"      0.3027 

"     "  =  14.25 

• 

.0         •'     CuS04,5H2O 

"      0.2533 

"    Cu  =  25.33«[ 

.0 

"      0.2533 

"     "  =25.33 

.0 

"      0  2534 

"     "  =25.34 

.0 

"      0.2537 

"     "  =2537 

.9210    "    SnCl4  +  2NH4Cl 

"      0.6219 

"     Sn  =  32.37**- 

< 

3.1320    " 

"      0.6900 

"     "  =32.36 

*  Classen's  method  :  see  Tiu,  later. 

f        "  "  "    Antimony,  later. 

\        "  "  "   Nickel 

§  "  "    Cobalt 

I        "  "  "    Iron 

T  From  the  acid  double  oxalate,  Classen's  method. 
**  From  the  acid  ammonium  double  oxalate. 


SECONDARY   BATTERIES. 

THIRD  DAY. 

Tension  1.95  volts.     (Voltameter  =  40  cc.  OH  gas. 
(Six  simultaneous  analyses.) 


1.0050  gm.  CuSO4,5H2O 

1.0170    " 

1.0006    " 

1.0013    " 

1.5680    "    SnCl4 +2NH4C1 

2.4520    " 


gave  0.2550gra.  Cu  =  25.37£ 

"  0.2580    "  "  =25.36 

"  0.2539    "  "  =25.37 

"  0.2540    "  "  =25.37 

"  0.5070    "  Sn  =  32.34 

"  0.7946    "  "  =32.40 


FOURTH  DAY. 


Tension  1.95  volts.     (Voltameter  =  40  cc.  OH  gas.) 


gave    0.2532  gm.  Cu  =  25.32# 


1.0 

gm. 

CuSO4,5H2O 

1.0 

« 

<  « 

1.0 

" 

«< 

1.0 

« 

« 

1.0 

ii 

" 

1.0 

K 

ii 

1.0 

" 

«< 

1.0 

«< 

" 

2.20 

ii 

NiS04+(NH4)2S04,6H20 

2.45 

« 

" 

2.1340 

" 

CoS04  4-  K2S04,6H2O 

2.4350 

ii 

«                      <  t 

0.2535 

"     "  =25.35 

0.2532 

"     "  =  25.32 

0.2536 

"     "  =25.36 

02535 

"     "  =25.35 

02538 

"     "  =25.38 

0.2539 

"     "  =25.39 

0.2537 

"     "  =25.37 

0.3277 

"    Ni  =  1489 

0.3650 

"     "  =14.89 

0.3148 

"   Co  =  14.75 

0.3587 

11   "  =i4.?a 

FIFTH  DAY. 
Tension  1.95  volts.    (Voltameter  =  40  cc.  OH  gas.) 


1.0 
1.0 

2.4120 
2.2130 


gm.  CuSO4,5H2O 
"     FeS04  -f  (NH4)2S04,6H2O 


gave  0.2537  gra.  Cu  =  25.37& 
"      0.2537    "     "  =2587 
"      0.3438    "    Fe  =  14.25 
"      0.3156    "     "  =14.26- 


a 

u 

0.2534 

" 

u 

— 

25 

.34 

« 

u 

0.2536 

" 

u 

r= 

25 

.36 

« 

a 

0.2533 

u 

« 

rrz 

25 

.33 

a 

u 

0.2537 

(I 

a 

= 

25 

.37 

" 

u 

0.2534 

u 

tt 

= 

25 

.34 

« 

u 

0.2536 

u 

" 

= 

25 

.36 

U 

a 

0.2535 

a 

u 

•  — 

25 

.35 

44  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

SIXTH  DAY. 

Tension  1.92  volts.     (Voltameter  =  39  cc.  OH  gas.) 
(Eight  simultaneous  copper  determinations.} 

1.0  gm.  CuSO4,5H2O    gave  0.2533  gm.  Cu  =  25. 

1.0  " 

1.0  " 

1.0  « 

1.0  " 

1.0  " 

1.0  " 

1.0  « 


Fifty  determinations,  it  is  seen,  were  made  in  six  days.  At 
the  end  of  the  sixth  day  the  tension  had  fallen  to  1.85  volts  and 
the  volume  of  OH  gas  to  37  cc.;  the  accumulators  were  therefore 
fully  charged  by  a  10-ampere  current,  receiving  an  addition 
of  54  ampere-hours.  Since  the  total  capacity  of  such  an  accu- 
mulator is  over  250  ampere-hours,  it  is  safe  to  assume  that  60 
to  TO  determinations  may  be  made  from  one  charge;  and  the 
experience  of  years  confirms  this  assumption. 

To  ascertain  whether  accumulators  in  use  still  retain  electric 
energy,  their  tension  may  be  measured,  or  the  specific  gravity 
of  the  sulphuric  acid  in  the  accumulator  may  be  determined, 
as  this  is  higher  when  the  accumulator  is  charged. 

Still  another  advantage  in  the  use  of  accumulators  is  found 
in  the  beauty  of  the  precipitated  metal,  resulting  from  the 
constancy  of  the  current,  which  much  exceeds  that  from  a 
galvanic  battery  or  a  dynamo.  In  the  Aachen  laboratory  two 
pairs  of  accumulators  have  been  constantly  used  since  1888, 
without  need  of  repair.  Two  accumulators  are  in  use  in  the 
analytical  laboratory,  for  which  they  have  proved  entirely 
sufficient,  and  two  in  the  author's  private  laboratory.  The 


SECONDARY    BATTERIES.  45 

current  from  the  dynamo  is  used  only  to  charge  the  accumu- 
lators. F.  Riidorff  *  has  recently  advocated  the  use  of  Mei- 
dinger  cells  in  place  of  the  "  much-extolled  dynamo,  with  or 
without  accumulators,"  f  on  the  ground  that  the  outfit  of 
dynamo  and  accumulators  is  attainable  only  by  laboratories 
commanding  abundant  supplies  and  the  use  of  power,  while 
even  then  the  necessary  attention  to  the  source  of  power  and 
the  dynamo  is  irksome.  As  to  the  use  of  Meidinger  cells, 
those  who  are  familiar  with  the  earlier  editions  of  this  work 
and  the  contributions  of  the  author  to  the  "Berichte"  are 
aware  that  he  early  recommended  the  use  of  Meidinger  bat- 
teries for  the  determination  of  many  metals,  made  investiga- 
tions as  to  the  strength  of  current  yielded  by  them,  and  used 
them  for  years  in  the  laboratory  under  his  charge.  These 
cells  are  entirely  satisfactory  when  they  are  not  needed  daily 
for  a  long  time  (e.g.,  in  small  commercial  laboratories),  and 
when  there  are  not,  as  a  rule,  a  large  number  of  determinations 
to  be  simultaneously  made.  This  has  been  established  by  the 
experience  of  many  former  pupils  of  the  author  who  have 
worked  or  still  work  with  Meidinger  cells.  The  use  of  Meid- 
inger cells  is,  however,  not  to  be  recommended  in  labora- 
tories for  instruction,  since  complete  precipitation  of  the 

*  Zeit.  fiir  angewandte  Chemie,  1892,  S.  3. 

f  It  is  admitted  that  the  author  first  suggested  the  use  of  the  dynamo 
and  accumulator.  His  introduction  of  the  dynamo  into  the  chemical  lab- 
oratory has  been  recognized  by  highest  authority  as  a  noteworthy  service 
(Kiliani  :  Berg-  u.  Htittenm.  Zeit.,  March,  1886).  The  author  is  unaware 
that  he  has  "extolled  "  these  sources  of  the  galvanic  current  ;  his  publica- 
tions have  simply  stated  their  advantages  over  galvanic  batteries,  giving 
results  of  experiment.  Dynamos  with  or  without  accumulators  have  been 
introduced  not  only  in  laboratories  for  instruction,  but  also  in  laboratories 
of  factories  and  smelting-works.  [Here  followed  a  list  of  laboratories, 
which  it  is  not  necessary  to  reproduce.  It  includes  only  Cornell  Univer- 
sity in  this  country.  —  Trans.} 


46  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

metal  requires  a  long  time  (12  to  14  hours),  arid  the  pupils  can- 
not watch  the  reaction.  For  this  reason  it  is  not  desirable  in 
a  laboratory  for  instruction  to  leave  solutions  for  analysis  to 
the  action  of  the  current  during  the  night.  Moreover,  while 
the  separation  of  certain  metals  (e.g.,  antimony  from  tin,  and 
arsenic,  cadmium,  bismuth,  or  mercury  from  other  metals)  is 
possible  with  Meidinger  batteries,  the  separation  of  many 
other  metals  is  attended  with  great  difficulties. 

The  possibility  of  conducting  analyses  during  the  night  is  a 
recognized  advantage  of  electrolysis;  cases  often  occur,  how- 
ever, in  technical  work  in  which  rapidity  is  equally  as  impor- 
tant as  accuracy,  both  for  control  of  manufacturing  processes, 
and  in  the  purchase  of  foreign  ores.  Now  Meidinger  cells 
are  totally  unsuited  to  rapid  analytic  work.  According  to  the 
manager  of  the  largest  smelting  works  in  the  Rhine  provinces 
and  Westphalia,  it  is  the  custom,  especially  in  foreign  markets, 
to  send  samples  of  ores  that  are  to  be  sold  at  auction  to  such 
smelting-works  as  use  them.  A  rapid  and  exact  determination 
of  the  metal  in  the  ore  is  of  the  highest  importance,  inasmuch 
as  the  auction  sales  take  place  without  delay,  and  the  value  of 
the  ores  to  the  smelting- works  must  be  promptly  given  to  the 
seller.  Thus  it  has  been  demonstrated  to  the  author  that  his 
methods  and  apparatus  first  made  it  possible  to  secure  a  rapid 
and  exact  determination  of  metals  in  such  cases,  and  that  the 
application  of  electrolytic  methods  is  adapted  to  greatly  facili- 
tate dealing  in  antimony,  lead,  silver,  gold,  and  other  ores. 
In  the  Aachen  laboratory,  in  which  determinations  were 
formerly  made  with  Meidinger  cells,  there  have  been  made 
for  some  years  past,  by  the  use  of  two  accumulators,  some 
2500  electrolytic  determinations  yearly,  on  the  average. 

Riidorff s  contention  that  only  such  laboratories  as  have 
large  resources  and  power  at  command  can  afford  to  use  tiie 
dynamo  with  or  without  acc'nmulators,  is  not  well  founded. 


\ 
SECONDARY    BATTERIES.  47 

A  small  dynamo  and  two  accumulators  can  be  obtained  for  a 
few  hundred  marks,  and  as  to  the  question  of  power,  Kudorff 
was  considering  only  laboratories  for  instruction.  In  indus- 
trial establishments  (as  well  as  in  the  Aachen  laboratory)  the 
dynamo  is  run  by  a  belt  from  shafting,  and  the  portable 
accumulators  are  charged  without  difficulty.  The  larger 
factories  usually  have  their  own  electric-lighting  plants,  the 
generator  of  which  can  conveniently  be  used  in  the  daytime 
to  charge  accumulators.  This  method  is  followed  in  a  smelt, 
ing  establishment  near  the  Aachen  laboratory.  In  cities 
Avhich  have  electric-lighting  plants  the  use  of  the  current  can 
be  obtained  for  charging  accumulators,  reducing  the  original 
outlay  to  the  purchase  of  two  accumulators.  Physicians  often 
use  accumulators  which  are  charged  by  the  local  electric-light- 
ing company.  In  any  case,  accumulators  may  be  charged 
from  Bunsen  cells;  there  is  needed  only  an  amperemeter  and 
a  resistance  so  as  to  insure  the  maximum  current  needed  for 
the  accumulators  in  use. 

[The  rapidity  with  which  Bunsen  cells  "  run  down  "  makes 
it  inconvenient  to  use  them,  as  suggested  in  the  last  paragraph, 
to  charge  secondary  batteries.  The  durability  and  uniformity 
under  constant  discharge  of  the  copper-sulphate  cell  suggests 
the  use  for  this  purpose  of  a  battery  of  special  large  cells  of  a 
simple  "  gravity  "  type,  such  as  could  be  put  up  in  any  labora- 
tory, using,  e.g ,  as  containers,  the  large  pails  or  small  tubs,  of 
some  kind  of  papier-mache  or  compressed  wood  fibre,  which 
are  to  be  found  in  any  grocery,  after  coating  them  heavily  on 
the  inside  with  asphalt  paint.  A  series  of  careful  experi- 
ments recently  conducted  with  the  knowledge  of  the  translator 
on  the  charging  of  a  secondary  battery  with  a  gravity  battery 
of  such  type  gave,  however,  very  indifferent  results. 

The  translator  believes  the  most  economical,  convenient, 
and  permanent  source  of  current  for  electrolytic  work,  in 


48  QUANTITATIVE    ANALYSIS   BY    ELECTROLYSIS. 

an  isolated  laboratory,  whether  for  research,  for  instruction,, 
or  for  technical  purposes,  to  be  some  form  of  the  Paget 
thermopile  described  on  p.  21  ff.,  with  a  secondary  battery  of 
two  cells.  Such  a  plant,  as  has  been  indicated,  can  be,  if  de- 
sirable or  necessary,  entirely  home-made. —  Trans .] 


APPARATUS  FOR  REDUCING  THE   STRENGTH 
OF  THE   CURRENT. 

RESISTANCES. 

In  order  to  obtain  currents  of  any  desired  strength  from  a 
galvanic  battery  or  an  accumulator,  it  is  often  necessary  to  place 
in  the  current  a  resistance.  As  appears  hereafter,  currents 
of  very  different  strengths  are  required  to  separate  different 
metals  from  their  solutions  in  a  manner  adapted  to  quantitative? 
analysis.  For  instance,  the  determination  of  iron  requires  a: 
current  of  0.5  to  1  ampere  ;  of  tin,  one  of  about  £  ampere;  of 
antimony,  one  of  about  0.15  ampere.  Constant  currents  of 
about  0.15  ampere  may  be  obtained  by  the  use  of  five  or  six. 
Meidinger  cells,  while  it  is  difficult  with  this  type  of  cells  to> 
obtain  or  to  maintain  constant  currents  of  0.5, 1,  or  1.5  amperes.. 
To  carry  on  a  number  of  antimony  or  other  determinations  by 
the  use  of  a  Meidinger  battery  requires  a  large  number  of 
cells,  while  any  desired  determination  or  separation  may  be 
carried  on  by  the  use  of  Bunsen  cells  or  accumulators  with 
suitable  resistances. 

The  author  commonly  uses,  for  the  reduction  of  the 
strength  of  a  current,  plug  rheostats.  As  ordinarily  con- 
structed, these  are  ill  adapted  to  laboratory  use,  for  the  plugs 
are  quickly  attacked  by  acid  vapors  from  the  cells,  or  the 
vapors  of  the  laboratory,  and  the  resistance  introduced  is 
thus  changed.  The  ordinary  apparatus  has  also  the  fault 


REDUCING   THE   STRENGTH   OF   THE   CURRENT.          49 

that  the  plugs  are  liable  to  become  loose.     Both  difficulties 
are  met  by  the  use  of  mercury  contacts,  instead  of  plugs,  to 


FIG.  29. 


FIG.  30. 


50 


QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 


connect  the  metallic  plates.  Fig.  29  shows  the  arrangement 
of  such  a  rheostat.*  By  inserting  the  contact  bars  CC  in 
the  mercury  cups  or  removing  them,  any  resistance  from  0.5 
to  80  ohms  may  be  inserted  by  intervals  of  0.5  ohms. 

The  following  results  of  experiment  show  the  action  of 
this  rheostat.  A  current  from  three  Bunsen  cells  yielding 
in  the  voltameter  28  cc.  oxyhydrogen  gas  per  minute  was 
reduced  as  follows  :  — 


Ohms  inserted. 

CC.  Oxyhydrogen 
Gas  per  Minute. 

Ohms  inserted. 

CC.  Oxyhydrogen 
Gas  per  Minute. 

0.5 

16.00 

15.0 

2.20 

1.0 

12.50 

20.0 

1.30 

1.5 

9.75 

30.0 

1.10 

2.0 

7.00 

40.0 

0.80 

3.0 

6.00 

50.0 

0.70 

4.0 

5.00 

60.0 

0.60 

5.0 

4.90 

70.0 

0.50 

7.5 

4.00 

80.0 

0.45 

10.0 

3.50 

A  current  of  16  cc.  oxyhydrogen  gas  pel*  minute  (yielded 
by  two  Bunsen  cells)  was  reduced  by  40  ohms  to  0.4,  and  by 
80  ohms  to  0.15  cc.  oxyhydrogen  gas. 

More  recently  the  author  has  used  the  simplified  form  of 
rheostat  shown  in  Fig.  30,  in  which  brass  plates  are  dispensed 
with,  and  the  contact  with  the  German-silver  coils  is  made 
directly  by  mercury. 

The  following  results  of  experiment  show  how  constant 
is  the  current  from  Bunsen  cells  when  a  rheostat  is  used.  In 
the  separation  of  antimony  from  tin,  the  current  from  two 


*  This  rheostat  is  made,  at  the  author's  suggestion,  by  Fraas  Brothers  in 
Wunsiedel. 


REDUCING  THE   STRENGTH   OF  THE  CURRENT. 


51 


Bunsen  cells  was  reduced  to  0.6  and  2  cc.  oxyhydrogen  gas 
per  minute. 

Columns  A  and  B  give  the  strength  of  the  two  Bunsen 
elements ;  columns  C  and  D,  that  obtained  by  use  of  the 
rheostat. 

A  and  C  were  measured  before  the  experiments;  B  and 
D,  after  them  (lapse  of  time,  14  hours). 


A. 

B. 

C. 

D. 

CC.  OH  Gas. 

CC.  OH  Gas. 

CC  OH  Gas. 

CC.  OH  Gas. 

17 

16.0 

0.6 

0.3 

24 

19.0 

0.6 

0.4 

18 

11.5 

0.6 

0.3 

17 

15.5 

0.6 

0.4 

With  a  view  to  economy  in  battery  power,  and  especially 
in  rheostats,  the  author  has  constructed  a  simple  apparatus 


FIG.  31. 


{Fig.  31)  which  allows  a  number  of  determinations  to  be 
carried  on  with  the  same  battery. 


52  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

The  current  from  the  battery  enters  at  «,  circulates  through 
the  German-silver  resistance  !N",  and  returns  to  the  battery 
through  b.  In  making  electrolytic  determinations  the  plati- 
num dishes  serving  as  negative  electrodes  may  be  attached  to 
any  one  of  the  binding-screws  1-20,  while  the  platinum  foil 
serving  as  positive  electrode  is  attached  to  the  binding-screw 
marked  with  the  +  sign.  The  apparatus,  therefore,  is  suited 
to  carry  on  eight  different  determinations  simultaneously. 
Its  value  for  analytical  purposes  is  shown  by  the  following 
experiments.  To  determine  directly  the  current  strength  at 
the  binding-screws  1-20,  150  cc.  of  a  15$  copper  sulphate 
solution  was  placed  in  each  of  6  platinum  dishes  of  equal 
size,  copper  electrodes*  were  used,  and  the  current  passed 
for  7  minutes  in  each  case. 

The  current  was  produced  by  a  battery  of  5  Bun  sen  cells, 
which  gave  56  cc.  OH  gas  in  the  first  and  second  experiments,, 
and  68  in  the  third. 


FIEST  EXPERIMENT. 

Gm.  Cu.     Amperes. 

Binding-screw  1 0.5064  =  3.75 

"  2  .*..»••  0.3507  =  2.617 

"  3  .    £-  .    .     .  0.2873  =  2.085 

"  4  .*  .....  0.2358  =  1.711 

"  5  .    >    .    .     .  0.1857  =  1.348 

"  6 0.1453  =  1.054 

"  7 0.1341  =  0.973 

"  8  0.1128  =  0.818 


*  6  cm.  in  diameter,  2  mm.  thick.    The  diameter  of  the  platinum  dishes 
was  9  cm.,  the  distance  of  the  electrodes  from  each  other  2.5  cm. 


REDUCING  THE   STRENGTH    OF  THE   CURRENT.          53 

SECOND  EXPERIMENT. 

Gin.  Cu.     Amperes. 

Binding-screw    7  .     .     .     .     .  0.2213  =  1.606 

"  8 0.1622  =  1.177 

«  9  .....  0.1356  =  0.984 

"  10 0.1083  =  0.786 

"  11  .    ,    .    ,    .  0.0846  =  0.614 

«  12 0.0744  =  0.576 

«  13 0.0506  =  0.367 

«  14 0.0410  =  0.225 

THIED  EXPERIMENT. 

Gm.  Cu.     Amperes. 

Binding-screw  13  .....  0.1983  =  1.446 

«  14 0.1304  =  0.946 

"  15 0.1276  =  0.926 

"  16 0.0855  =  0.620 

"           17  .....  0.0605  =  0.439 

«  18 0.0385  =  0.280 

"  19 0.0314  =  0.227 

"  20 0.0136  =  0.098 

From  a  number  of  quantitative  determinations,  which, 
were  made  by  Norrenburg  with  this  apparatus,  the  following 
are  selected :  — 

SERIES  I. 

i 

The  apparatus  was  attached  to  a  battery  of  5  Bunsen  cells, 
yielding  62  cc.  OH  gas,  and  eight  iron  determinations  made 
simultaneously.  The  precipitation  was  complete  in  6  hours. 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


Taken 
FeSO4,  2(NH4)2SO4.6H2O. 

1.2918  gm. 
1.4360     " 
1.1926     " 

1.1964  « 
1.2945  " 
1.3218  « 

1.2931  " 
1.3255  " 


Found 
Fe. 


0.1846  gm.  =  14.30$  ] 

0.2059  "  =  14.33     I 

0.1708  «  =14.32    j 

0.1700  «  =14.30 

0.1851  «  =14.30 

0.1892  "  =  14.31 

0.1854  "  =  14.34 

0.1895  "  =14.30 


Binding         Cc. 
Screw.      OH  Gas, 

24.0 

25.0 
24.0 

16.8 
16.6 
17.2 

13.2 


At  the  close  of  the  experiment  the  battery  (without  resist- 
ance) still  yielded  50  cc.  OH  gas. 


SERIES  II. 

Three  nickel  and  five  copper  determinations  were  con- 
ducted simultaneously.  The  current  from  5  Bunsen  cells- 
yielded  65  cc.  OH  gas. 


Taken 
Nickel  Ammon.  Sulph. 

1.2848  gm. 
1.4341    « 
1.2008    " 


Found 
Nickel. 

0.1963  gm.  =  15. 
0.2201    "     =  15.35 
0.1842   «     =  15.34 


Binding     Cc. 
Screw.   OH  Gas. 


Copper  Sulphate. 
1.1531  gm. 

0.9787    " 
1.0092    « 

0.9938    " 
1.0088    " 


Copper. 

0.2910  gm.  =  25. 
0.2476     "    =  25.30 
0.2556     "    =  25.32 

0.2515     "    =25.30 
0.2550     «    =25.27 


DEDUCING   THE   STRENGTH    OF   THE   CURRENT. 


SERIES  III. 

This  established  the  applicability  of  the  process  to  the 
simultaneous  determination  of  nickel,  antimony,  and  copper. 
The  number  of  analyses  again  was  eight.  The  battery,  of 
5  Bunsen  cells,  yielded  65  cc.  OH  gas  per  minute. 

Taken 
Nickel  Ammonium  Sulphate. 

1.3022  gm. 
1.1520     " 
1.4391     " 

Antimony  Tersulphide. 
0.1609  gm. 
0.1691     " 
0.1626     « 
Copper  Sulphate. 
0.2527  gm. 
0.2550     « 

The  current  strength  of  the  battery  at  the  close  of  the  last 
two  series  was  about  half  that  at  the  beginning. 

These  experiments  show  plainly  the  practical  advantage 
of  the  new  rheostat.  To  perform  eight  iron  determinations 
(Series  I.)  simultaneously  without  this  rheostat  would  require 
8  ordinary  rheostats  and  at  least  16  cells.  For  three  nickel 
and  five  copper  determinations  would  be  needed  16  Bunsen 
cells  and  8  rheostats,  or  6  cells,  3  rheostats,  and  5  Meidinger 
batteries  of  3  or  4  cells  each,  the  latter  for  the  copper  deter- 
minations. The  conditions  would  be  similar  with  the  third 
series. 

[Prof.  Edgar  F.  Smith  uses  a  simple  apparatus  (Fig.  32), 
the  accompanying  figure  and  description  of  which  he  kindly 
permits  the  translator  to  copy  : 

"The  writer  has  for  some  time  employed  a  much  simpler 


Found 

Binding 

Cc. 

Nickel. 

Screw. 

OH  Gas. 

15.30$) 

(  21.0 

15.27     L 

3 

-j  22.0 

15.32     j 

(  22.0 

Antimony. 

71.44$) 

(    1.0 

71.47    I 

9 

\  1.0 

71.49    ) 

(    1.0 

Copper. 

25.30     | 
25.30     J 

7 

)    3.6 
)    3.6 

56  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

current-reducer,  which  has  the  advantage  of  cheapness  and 
ready  construction  to  recommend  it.  It  consists  of  a  light 
wooden  parallelogram,  about  6  feet  in  length.  Extending 
from  end  to  end,  on  both  sides,  is  a  light  iron  wire,  measuring 


FIG.  32. 


in  all  about  500  feet.  With  the  binding-posts  at  a  and  5,  and 
a  simple  clamp,  it  is  possible  to  throw  in  almost  any  resistance 
that  may  be  required.  It  answers  all  practical  purposes,"  — 
"  Electro-chemical  Analysis,"  p.  29.  —  TransJ] 


MEASUREMENT    OF    STRENGTH    OF    THE    CURRENT.          57 
MEASUREMENT    OF  THE    STRENGTH    OF  THE    CURRENT. 

The  unit  of  strength  is  the  ampdre,  that  of  electro-motive 
force  is  the  volt,  that  of  resistance  is  the  ohm.  In  a  current, 
the  resistance  of  which  is  1  ohm,  a  current  of  1  ampdre  gives 
the  force  of  1  volt.  1  ohm  is  equal  to  1.065  Siemens  units, 
;S.  U.  (a  column  of  mercury  1  m.  long  and  1  sq.  mm.  in  area  of 
.section,  at  0°) ;  1  volt  =  0.95  Daniell  (D.  =  the  electro-motive 

force'  of  a  Daniell  cell)  ;  1  ampere  =  * ;  1  ampere  = 

1  o.  LJ. 

10.436  cc.  oxy hydrogen  gas  at  0°  and  760  mm.  pressure 
=  19.69  mg.  copper  •=  67.1  mg.  silver  in  1  minute.  The 
strength  of  current  in  cc.  of  oxyhydrogen  gas,  multiplied  by 
0.0958,  gives  the  strength  in  amperes. 

For  the  measurement  of  the  strength  of  the  current,  either 
the  chemical  or  the  magnetic  action  of  the  current  is  used. 
In  the  former  case,  the  strength  of  the  current  is  determined 
by  the  galvanic  decomposition  of  water,  and  measurement  of 
the  volume  of  oxyhydrogen  gas  produced.  A  special  appar- 
atus, the  voltameter,  is  used  for  this  purpose  ;  its  construction 
is  shown  in  Fig.  33.  The  cylindrical  vessel  g  is  partly  filled 
with  pure  dilute  33  per  cent  sulphuric  acid.  The  platinum 
wires  d  and  d'  welded  to  the  platiiium  strips  p  and  p^*  are 
fused  into  the  walls  of  the  vessel  g.  This  latter  stands  in  a 
large  cylinder  C  of  water  which  serves  to  cool  it.  The 
platinum  wires  end  in  the  screws  s  and  «',  which  are  con- 
nected with  the  battery.  The  oxyhydrogen  gas,  as  it  is 
formed,  passes  through  the  tube  r,  which  contains  a  little 
water,  and  then  is  collected  in  the  measuring-tube  R,  which 
is  graduated  into  •£-§  cc.,  and  filled  with  water.  To  measure 
an  electric  current  with  the  voltameter,  the  water  over  which 


*  In  the  apparatus  used  by  the  author,  the  platinum  electrodes  are 
31  X  13  mm.,  and  are  distant  from  each  other  20  mm. 


58  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  gas  is  collected  is  first  saturated  with  oxy hydrogen  gasr 
and  then,  by  the  use  of  a  watch  with  second-hand,  the 
volume  of  gas  is  observed,  which  the  current  yields  in  a 


FIG.  33 

minute,  or,  if  the  current  is  weak,  in  a  longer  time.  To 
compare  observations,  the  volume  should  be  reduced  to  0° 
and  760  mm.  pressure. 

v   =.  observed  volume  of  oxyhydrogen  gas. 

vl  =  normal  volume  (at  0°  and  760  mm.). 

t    =z  observed  temperature. 

h  =  pressure  reckoned  in  mm.  of  mercury. 

v  h 

Vl  "  1  +  0.00367*  '  760' 


MEASUREMENT   OF   STRENGTH   OF   THE   CURRENT.      50 

Let  I  indicate  the  height  of  the  column  of  liquid,  s  the 
density  of  the  liquid,  and  I  the  barometric  height ;  then 


The  foregoing  long-used  form  of  voltameter  is  inconvenient 
in  use,  and,  moreover,  admits  of  no  allowance  for  pressure  in 
reading  off  the  volume  of  gas.  Unless  -the  measuring-tube  is 
completely  filled  with  water  before  each  experiment,  the  re- 
sults from  the  same  current  will  differ  according  to  the  quan- 
tity of  OH  gas  which  the  eu- 
diometer already  contains.  J. 
Walter  has  constructed  an  ap- 
paratus for  electrolytic  analysis 
which  is  much  more  convenient 
than  the  preceding,  and  is  also 
free  from  the  fault  above  men- 
tioned. Walter's  form  of  the 
apparatus  cannot,  however,  be 
used  in  connection  with  this 
work,  as  the  author's  observa- 
tions are  all  based  on  the  use 
of  a  voltameter  with  platinum 
electrodes  31  mm.  long,  13 
rnm.  wide,  and  20  mm.  apart. 
G.  Neumann,  former  assistant 
in  the  Aachen  laboratory,  has 
therefore  modified  Walter's  ap- 
paratus to  the  form  shown  in 
Fig.  34.  This  voltameter  con- 
sists of  a  tube  A  containing  40 
cc.  and  graduated  into  -fa  cc.,  the  graduation  beginning  below 

*  13.6  =  sp.  gr.  of  mercury. 


FIG.  34. 


60  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  cock  a.  Below  the  graduation  the  tube  is  so  widened  that 
the  two  platinum  electrodes,  31  ram.  long  and  13  rnm.  wide, 
can  readily  be  placed  at  the  required  distance  from  each  other. 
Below  the  cylindrical  vessel  c  thus  formed  the  tube  is  again  nar- 
rowed for  the  attachment  of  a  rubber  tube  connecting  it  with 
the  pressure-tube  B.  The  wires  leading  from  the  electrodes 
are  fused  into  the  walls  of  <?,  and  then  coiled  as  shown  in  the 
figure.  It  is  desirable  to  mount  the  apparatus  on  a  stand, 
as  shown.  The  voltameter  is  tilled  with  33$  sulphuric  acid, 
and  the  acid  brought  to  the  0  mark  by  raising  tube  B  and 
opening  cock  a.  Any  acid  that  passes  the  cock  is  held  in  the 
small  funnel  b.  After  a  is  closed,  the  electrodes  are  connected 
with  the  source  of  current,  and  OH  gas  is  given  off.  After 
the  current  has  acted  for  the  exact  time  desired,  it  is  broken, 
and  the  gas  measured  after  equalizing  the  level  of  the  acid  in 
tubes  A  and  B.  Before  equalizing  the  level  the  apparatus 
should  be  lightly  shaken,  and  allowed  to  stand  until  all  gas 
bubbles  have  risen  into  tube  A. 

The  other  method  of  measuring  the  current  depends  on 
the  deflection  of  the  magnetic  needle  by  the  current,  and 
requires  the  use  of  a  galvanometer  (tangent  galvanometer, 
sine  galvanometer,  or  other  form  of  instrument). 

In  order  to  express,  in  terms  of  chemical  action,  the 
deflection  of  the  needle,  it  is  placed  in  the  same  current  with 
a  voltameter,  and  the  deviation  of  the  needle  is  observed,  as 
well  as  the  volume  of  oxyhydrogen  gas  (reduced  to  0°  and 
760  mm.)  which  is  produced  in  a  minute.  Placing  the. 

/y 

volume  =  v,  the  quotient gives  the  standard  value  for 

the  galvanometer.*     If  this  standard  value  is  denoted  by  R, 

*  The  strength  of  a  current  that  produces  1  cc.  oxyhydrogen  gas  in  1 
minute  is  called  1.  A  current  that  yields  «  cc.  in  a  minute  has  the  strength 


MEASUREMENT   OF   STRENGTH    OF   THE   CURRENT.      61 


the  strength  J  of  a  current  which  produces  the  deviation  a  is 

J  =  R  tan  a. 

If,  for  instance,  with  both  voltameter  and  galvanometer 
in  circuit,  12.3  cc.  (v)  oxyhydrogen  gas  are  produced  in  a 
minute,  and  the  deviation  of  the  needle  (a)  is  15°  30',  then 


K  = 


12.3 


tan  15°  30 


7  =  44.35. 


If,  in  another  experiment,  the  same  galvanometer  shows 
a  deviation  (a^  of  25°,  the  strength  J  of  the  current,  in 
chemical  terms,  is 

J  =  44.35  tan  25°  =  20.68.* 

The  method  of  measuring  the  current  with  a  Siemens 
torsion  galvanometer  is  indicated  on  p.  32. 

The  Kohlrausch  amperemeter,  recommended  on  p.  41,  is 
specially  adapted  for  measuring 
the  current  strength.  It  is  made 
by  Hartmann  u.  Braun  of  Bock- 
enheim,  Frankfurt  a.  M.,  in  dif- 
ferent sizes.  An  instrument  with 
a  scale  graduated  from  0  to  2 
amperes,  thus  showing  small  frac- 
tions of  an  ampere,  is  in  use  in  the 
Aachen  laboratory.  [The  author, 
although  thus  recommending  the 
Kohlrausch  amperemeter,  gives  no 
cut  of  it.  The  translator  is  enabled, 
by  the  courtesy  of  Prof.  Edgar  F. 
Smith,  to  supply  the  omission  from  FIG.  35. 

the  latter' s  work  on  Electro-Chemical  Analysis  (Fig.  35)]. 

*  A.  Winkelmann,  Physikalische  Lehren  (Vieweg,  Braunschweig). 


•62  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

It  has  been  the  custom  with  chemists  who  have  given 
their  attention  to  electrolytic  work  to  express  the  strength  of 
the  current  in  chemical  terms,  i.e.,  in  cc.  of  oxyhydrogen  gas, 
and  to  use. the  voltameter  almost  exclusively  for  measurement. 
It  is  well  known  that  the  voltameter  is  entirely  unsuited  to 
scientific  measurement,  because,  among  other  things,  it  has  a 
very  considerable  tension,  which,  under  certain  circumstances, 
may  be  greater  than  that  of  the  test.* 

Moreover,  all  the  measurements  given  in  the  earlier  litera- 
ture of  the  subject  are  made  upon  the  current  before  the 
substance  to  be  decomposed  is  placed  in  the  circuit,  and  not, 
as  plainly  should  be  the  case,  and  as  described  on  p.  33,  when 
both  the  voltameter  (or  galvanometer)  and  the  substance  to  be 
decomposed  are  in  the  circuit.  In  the  experimental  work 
described  in  the  following  pages  the  author  has  always  fol- 
lowed the  latter  method,  both  because  of  its  simplicity,  and 
especially  because  only  in  this  way  can  full  knowledge  be 
gained  of  the  strength  of  current  suitable  for  the  purposes  of 
quantitative  analysis.  As  will  appear  in  the  following  pages, 
a  current  of  a  certain  exact  strength  is  not  required  for  a 
quantitative  determination  or  separation ;  it  may  vary  within 
certain  bounds,  so  that,  in  most  determinations,  a  variation  of 
several  cc.  oxyhydrogen  gas  per  minute  is  of  no  influence. 
Only  for  this  reason  is  it  possible  to  conduct  analyses  by 
electrolysis  without  great  difficulty. 

The  statements  made  in  the  following  pages  as  to  strength 
of  current  are,  of  course,  true  only  when  the  experiment  is 
repeated  as  nearly  as  possible  under  the  same  conditions 
(size  and  form  of  electrodes,  distance  of  electrodes  from  each 
other,  concentration  of  solution,  etc.). 

*  The  comparison  of  results  with  the  voltameter  is  possible  only  when 
the  platinum  plates  are  of  the  same  size,  and  the  Mime  distance  from  each 
other,  and  the  acid  has  the  same  decree  of  concentration. 


MEASUREMENT   OP   STRENGTH   OF   THE   CURRENT.      63 

To  compare  electrodes  of  different  forms  it  is  necessary  to 
consider  not  only  the  current  strength,  but  also  the  current 
density  (i.e.,  the  relation  of  the  current  strength  to  the  surface 
of  the  electrode  used).  If  the  current  density  is  represented 
by  D,  the  current  strength  by  I,  and  the  surface  of  the  elec- 
trodes on  which  the  metal  is  deposited  by  S,  then  D  =  ~-. 

fe 

The  surface  must  then  be  accurately  determined.  If  the  same 
form  of  electrode  is  always  used,  e.g.,  a  platinum  dish  as  nega- 
tive electrode,  the  surface  can  be  easily  ascertained  by  filling 
with  a  liquid  to  varying  heights. 

For  the  platinum  dish  described  on  p.  65  (Fig.  36)  the 
following  relations  between  surface  and  contents  are  approxi- 
mately correct :  — 

Surface  in  Sq.  Cm.  Contents  in  Cc. 

63  50 

92  100 

113  150 

124  175 

In  the  laboratory  of  the  Munich  High  School  the  relation 
of  current  strength  to  surface  of  electrodes  has  been  deter- 
mined for  a  number  of  metals,  and  these  determinations  are 
used  in  various  places  in  this  work.  The  normal  current 
density  is  designated  by  N .  D]00 ,  reckoned  on  100  cc.  of  sur- 
face on  which  the  metal  is  deposited. 

For  any  surface  S  the  corresponding  current  strength  is 

S_ 
100* 

In  the  precipitation,  e.g.,  of  iron  from  a  solution  of  the 
ammonium  double  oxalate,  if  N .  D100  =  0.5  ampere,  then  if 

-1  Q  A 

S  =  180  the  current  strength  in  amperes  is  O-^-J-QA  =  0.9. 
All  the  results  of  the  Munich  laboratory  were  of  course  ob- 


reckoned  from  the  formula  I  =  "N" .  D100 .  77^. 


64  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

tained  when  both  the  amperemeter  and  the  dish  serving  as 
electrode  were  in  circuit. 

THE   PROCESS   OF   ANALYSIS. 

The  performance  of  a  quantitative  analysis  by  electrolysis 
requires,  above  all  things,  extreme  cleanliness.  As  it  is  impos- 
sible, in  electro-plating,  to  obtain  a  metallic  coating  on  any 
surface  which  is  not  most  carefully  cleaned  before  it  is  placed 
in  the  bath,  so  a  quantitative  analysis  cannot  be  successfully 
carried  out  unless  the  metallic  surface  serving  as  kathode  is 
previously  perfectly  cleaned  and  freed  from  fat.  The  same 
care  must  be  used  with  the  battery  connections,  the  stand 
which  serves  to  conduct  the  current,  etc. ;  otherwise  it  is- 
impossible  to  avoid  the  weakening  or  breaking  of  the  current. 

It  is  plainly  desirable  that  the  surface  of  the  kathode 
should  be  large  in  order  that  the  separated  metal  may  be 
more  firmly  attached  to  it.  If  a  metal  separates  from  a  solu- 
tion in  dense  form,  as  is  the  case  in  the  electrolysis  of  double 
oxalates,  the  possibility  of  the  oxidation  of  the  metal  is* 
scarcely  increased  by  enlarging  the  kathode. 

In  the  separation  of  peroxides  (e.g.,  lead  and  manganese 
peroxides),  which  are  much  less  firmly  attached,  the  size  of 
the  electrode  on  which  they  are  deposited  is  of  especial 
importance. 

It  is  not  desirable,  therefore,  to  employ  a  platinum  cruci- 
ble for  electrolytic  precipitation  if  more  than  a  few  milli- 
grams are  to  be  determined ;  not  only  is  the  surface  of  the 
kathode  too  small,  but  the  electrodes  are  not  widely  enough 
separated  to  facilitate  the  separation  of  the  metal  in  a  dense 
form. 

For  these  reasons,  the  author  uses  as  the  negative  electrode 
a  thin  platinum  dish  of  35-37  grams  weight,  9  cm.  diameter, 
4.2  cm.  depth,  and  about  225  cc.  capacity.  The  dish  has  the 


THE   PROCESS    OF   ANALYSIS.  65 

form  shown,  in  about  one-half  natural  size,  in  Fig.  36.  Dishes 
which  have  become,  in  the  course  of  time,  rough,  scratched, 
or  bent,  cannot  be  used  for  electrolysis. 

Some    metals   separate   less   readily  in  hammered   dishes 
than  in    those   which  are   spun   and    polished   on    the   lathe. 


FIG.  36. 

If,  for  instance,  hammered  dishes  are  used  for  the  reducl 
of  zinc  from  the  double  oxalate,  there  always  remains,  after 
the  solution  of  the  metal  in  acid,  a  gray,  closely  adherent 
coating  of  platinum  black,  which  is  with  difficulty  removed 
even  by  melted  potassium  hydrogen  sulphate,  and  which 
makes  further  determination  of  metals  in  the  dish  difficult. 
For  this  reason  it  is  desirable  to  use  perfectly  polished  and 
thoroughly  cleaned  dishes  for  electrolysis,  and  to  reserve  them 
exclusively  for  that  purpose. 

As  anode  (positive  electrode),  the  author  uses  a  plate  of 
moderately  thick  platinum  foil,  about  4.5  cm.  in  diameter, 
which  is  fastened  to  a  tolerably  stout  platinum  wire  (Fig.  37). 
It  is  desirable,  in  order  to  insure  uniformity  of  the  solution 
during  electrolysis,  to  make  a  few  holes  in  the  platinum  foil 
with  a  cork- borer.  If  this  is  neglected,  a  large  bubble  of  gas 
may  form  under  the  anode  by  the  union  of  several  smaller 
ones,  and  this  bubble,  on  escaping,  may  cause  spirting  and  loss. 

Yery  recently  the  author  has  used  as  positive  electrode  a 
platinum  dish  of  the  form  shown  in  Fig.  36,  50  mm.  in  diam- 
eter and  20  mm.  deep.  To  secure  better  circulation  of  the 
solution,  and  more  rapid  reduction,  the  electrode  has  five 


66 


QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 


openings  in  it.  This  form  of  electrode  is  specially  adapted 
to  the  determination  of  such  rnetals  as  have  the  tendency 
to  separate  in  a  floccnlent  state,  e.g.,  cadmium  and  bismuth. 

Two  standards  were  formerly  used,  as  shown  in  Fig.  45,  to 
support  the  two  electrodes.  The  author  substituted  a  single 
standard  (Fig.  38)  provided  with  a  metallic  ring  to  which 
three  short  contact  wires  of  platinum  are  riveted  for  the  plati- 
num dish  to  stand  on,  and  an  insulated  arm  a  of  glass  to  sup- 


FIG.  87.  Fiu    38. 

port  the  positive  electrode.  The  use  of  this  stand  has  the 
drawback  that  the  brass  rod  to  which  the  metallic  ring  and 
glass  arm  are  clamped  is  readily  corroded  by  the  laboratory 
vapors,  and  this  may  lead  to  the  breaking  of  contact.  The 
stand  shown  in  Fig.  39  has  given  good  service  for  a  long  time. 
King  and  arm  are  clamped  to  a  glass  rod  Gr,  and  n  is  connected 
with  the  negative  and  pt  with  the  positive  pole.  The  posi- 


THE   PROCESS   OF   ANALYSIS. 


67 


live  electrode  is  clamped  in  place  at  e.  If  a  platinum  cone 
is  used  instead  of  a  platinum  dish  for  the  deposition  of  the 
metal  (as  described  later),  two  arms  are  clamped  to  the  glass 
standard,  as  shown  in  Fig.  40.  This  arrangement  is  also 
convenient  when  a  metal  is  to  be  precipitated  from  an  acid 
solution ;  the  standard  with  the  electrodes  is  removed  quickly 


FIG.  39.  FIG.  40. 

from  the  solution  and  plunged  into  a  vessel  of  water,  and  the 
water  is  finally  removed  from  the  negative  electrode  by  wash- 
ing with  alcohol. 

[It  is.  of  course,  not  necessary  to  use  a  special  standard 
constituting  a  part  of  the  conductor,  as  shown  in  the  figures. 
The  platinum  dish  may  be  placed  on  the  table  or  the  base  of 
a  wooden  standard,  on  a  coil  of  platinum  or  bright  copper 
wire  which  is  connected  with  the  negative  pole  of  the  bat- 
tery ;  and  the  positive  electrode  may  be  held  in  a  wooden 
clamp  on  such  a  standard,  and  connected  directly  with  the 
•wire  from  the  positive  pole.  See  also  pp.  70-73. — Trans.] 

When  a  platinum  dish  is  used  it  may  be  placed  on  a  metal- 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS, 


FIG.  42. 


FIG.  43. 


FIG.  44. 


THE   PROCESS   OF   ANALYSIS. 


69 


lie  tripod  in  a  beaker,  and  the  acid  displaced  by  a  stream  of 
water  from  a  wash- bottle  after  the  reduction  is  complete. 


FIG.  45. 

The  electrodes  used  almost  exclusively  for  copper  deter- 
minations at  the  Mansfeld  smelting-works  are  shown  in  Figs. 
41  to  46.  According  to  the  quantity  of  metal  to  be  deter- 


FIG.  46. 


mined,  the  cylinder  of  platinum  foil  shown  in  Fig.  41  (one- 
half  natural  size),  or  the  platinum  cone  shown  in  Fig.  42 
(one-fourth  natural  size)  is  used.  The  positive  electrode  is 


70  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

either  a  thick  platinum  wire  wound  in  spiral  form  (Fig.  43)r 
or  has  the  form  shown  in  Fig.  44.  The  arrangement  of  the 
several  parts  is  shown  in  Figs.  45  and  46. 

[An  apparatus  described  by  v.  Malapert  *  is  specially 
adapted  to  the  use  of  the  above-described  electrodes,  and 
particularly  to  carrying  on  simultaneously  several  similar 
determinations. 

As  shown  in  Fig.  47,  a  single  wooden  standard  A  sup- 
ports the  apparatus  for  several  electrolytic  determinations, 

the  lower  board  B  carrying  the 
vessels  containing  the  solutions 
to  be  electrolyzed,  and  the 
upper  board  C  the  apparatus 
for  directing  the  current  as  de- 


FIG.  47.  FIG.  48. 

sired.    In  the  apparatus  described,  the  two  boards  are  18  cm. 
apart,  and  the  upper  board  7  cm.  wide. 

Fig.  48  shows,  on  a  larger  scale,  the  arrangement  for 
directing  the  current,  connected  with  each  pair  of  electrodes. 
The  two  strips  bb  of  brass  are  1  cm.  wide,  2  mm.  thick,  and 
their  centres  3  cm.  apart.  The  binding-screws  aa  serve  for 
the  attachment  of  the  electrodes ;  to  cc  are  attached  the 
coriducting-wires.  The  switch  d  establishes  or  breaks  con- 

*  Zts.  Anal.  Ch.,  26,  56. 


THE   PROCESS   OF   ANALYSIS.  71 

nection  between  the  two  strips  according  as  it  is  in  the  posi- 
tion shown  in  the  cut  (closed),  or  is  moved  to  bear  on  the 
curved  strip  of  hard  rubber  e,  (open). 

When  the  apparatus  is  arranged,  as  shown  in  Fig.  47,  with 
the  conducting- wires  from  the  battery  connected  with  the 
end  binding-screws,  and  adjacent  binding-screws  throughout 
connected  b}^  wires,  the  current  passes  unhindered  so  long  as 
the  switches  are  closed.  To  insert  any  desired  number  of 
similar  solutions  for  electrolysis,  it  is  only  necessary  to  place 
the  solutions  and  electrodes  in  position,  and  open  the  corre- 
sponding switches ;  the  current  is  then  forced  to  pass  through 
the  solutions. 

If  dissimilar  determinations  are  to  be  made,  the  connecting- 
wires  between  adjacent  pairs  of  brass  strips  are  removed,  and 
the  conducting-wires  from  each  battery  in  use  are  brought 
directly  to  the  binding-screws  cc  of  one  pair  of  strips. 

To  remove  acid  solutions  without  interrupting  the  cur- 
rent, v.  Malapert  uses  beakers  of  heavy  glass  8  cm.  in  diam- 
eter and  12  cm.  high,  with  a  side  tubulure  near  the  top,  as 
shown  in  Fig.  47.  A  cork  is  inserted  in  the  hole  between 
the  brass  strips  shown  in  Fig.  48,  through  which  passes  with 
little  friction  a  glass  tube  connected  by  rubber  tubing  with  a 
reservoir  of  water.  When  the  precipitation  is  complete,  a 
stream  of  water  is  turned  on,  and  the  acid  solution  displaced, 
passing  off  through  the  tubulure.  A  common  beaker  with 
siphon  can,  of  course,  be  used. 

A  resistance  coil  of  German-silver  wire  is  shown  in  Fig. 
47  connected  to  the  pair  of  binding-screws  at  the  extreme 
right.  Any  desired  resistance  can  be  thus  conveniently 
inserted. 

An  apparatus,  made  according  to  this  description,  was 
prepared  for  use  in  the  chemical  laboratory  of  the  Pennsyl- 
vania State  College,  with  an  addition,  devised  by  the  trans- 


72  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

lator,  which  makes  it  equally  convenient  when  a  platinum 
dish  is  used  as  the  negative  electrode. 

Fig.  49  shows  the  nature  of  the  addition  referred  to.  The 
brass  strip  connected  with  the  negative  electrode  is  extended 
downward,  at  the  rear  (G,  Fig.  49),  to  the  lower  board.  Here 
it  is  connected  with  the  brass  plate  H,  which  is  set  into  the 


FIG.  49. 

board  B  so  as  to  be  flush  with  its  upper  surface,  and  has  a 
shallow  saucer-shaped  depression,  the  centre  of  which  is 
directly  beneath  the  binding-screw  to  which  is  attached  the 
positive  electrode.  The  plate  H  and  the  entire  strip  were 
cut,  in  the  apparatus  originally  made,  from  a  single  sheet  of 
brass. 

A  platinum  dish  placed  in  the  saucer-shaped  depression  is 
firmly  supported,  and  is  in  good  metallic  connection  with  the 


THE   PROCESS    OF   ANALYSIS. 


73 


negative  pole  of  the  battery ;  the  positive  electrode  is  attached 
as  in  the  original  form  of  the  apparatus.  All  the  adjustments 
of  the  original  apparatus  are  retained,  and  the  brass  plate 
offers  no  impediment  to  the  use  of  a  beaker  with  cone-shaped 
negative  electrode,  as  shown  in  Fig.  47. —  Trans.~\ 


FIG.  50. 


FIG.  51. 


Herpin  uses,  for  electrolysis,  the  apparatus  shown  in 
Fig.  50.  The  platinum  dish  P  standing  on  the  tripod  F  is 
connected  with  the  negative  pole,  the  platinum  spiral  S 
(shown  separately  in  Fig.  51)  with  the  positive.  The  dish  is 
covered  with  a  glass  funnel  T  to  avoid  loss  by  spirting  of 
the  solution. 


74 


QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


Riche  uses,  as  kathode,  a  platinum  cone  (Fig.  52)  open 
at  both  ends,  having  the  form  of  a  crucible,  and  provided 
with  a  bail.  Oblong  openings  are  made  in  the  cone  to  facili- 
tate a  uniform  concentration  of  the  liquid  during  reduction. 
The  cone  is  placed  in  a  platinum  crucible  so  that  it  is  2  ta 
4  mm.  from  it.  The  whole  arrangement  is  seen  in  Fig.  53. 


FIG.  52. 


FIG.  53. 


To  return  to  the  consideration  of  the  actual  electrolysis: 
sulphates  are  best  adapted  to  conversion  into  double  ox- 
alates  (see  p.  6),  chlorides  less  so,  and  nitrates  entirely 
unadapted.  If  chlorides  have  been  used,  and  the  smell  of 
chlorine  is  observed  during  the  electrolysis,  ammonium  oxa- 
late  must  be  gradually  added  to  the  solution  till  the  odor 
disappears.  Sometimes  potassium  oxalate,  sometimes  ammo- 


THE   PROCESS   OF   ANALYSTS. 


75' 


nium  oxalate,  and  often  a  mixture  of  the  two,  is  used  for  the- 
preparation  of  the  double  salt. 

As  hot  solutions  conduct  the  current  more  readily,  solu- 
tions are  often  heated  before  they  are  submitted  to  electro- 
lysis. In  some  cases,  however,  as  in  the  determination  of 
antimony,  it  is  necessary  that  the  solution  should  be  of  the- 
ordinary  temperature. 

In  certain  determinations  and  separations,  it  is  best  to  keep 
the  solution  to  be  electrolyzed  at  moderate  heat,  not  above  50° 
C.  The  following  experiments  show  the  influence  of  heat  on 
the  time  required  for  electrolysis.  Nearly  equal  weights  of 
iron  and  nickel  were  precipitated,  under  as  equal  conditions 
as  possible  (strength  of  current,  concentration,  etc.),  from, 
solutions  kept  respectively  at  50°  and  15°:  — 

IRON. 


Taken. 

Found. 

Strength  of  Current. 

Time. 

CC.  OH  Gas. 

h.  m. 

j    <  0.2385  g.  Fe203 

(Cold)   . 

0.2384 

11 

4  20 

'  I  0.2345  g.  Fe2O3 

(Warm), 

0.2342 

11 

2  10 

n    (0.2246 

(Cold)   . 

0.2244 

10 

4  10 

'  C  0.2369 

(Warm), 

0.2369 

10 

2  15 

NICKEL. 


Taken. 

Found. 

Strength  of  Current. 

Time. 

CC.  OH  Gas. 

h.  m. 

j     (  0.2660  g.  Ni 

(Cold)    . 

0.2660 

13 

7  25 

'  I  0.2660  g.  Ni 

(Warm), 

0.2659 

13 

2  20 

n    (  0.2660  g.  Ni 

(Cold)   . 

0.2661 

13 

7  30 

'  I  0.2660  g.  Ni 

(Warm), 

0.2660 

13 

2  20 

It  also  results  from  the  foregoing  experiments,  that  the 
current-strength  can  be  greatly  reduced  by  the  use  of  hot  solu- 
tions, in  case  there  is  no  occasion  for  hastening  the  electrolysis. 


76  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

The  statements  in  this  book  apply  to  solutions  at  the 
ordinary  temperature,  except  when  the  contrary  is  stated. 

For  heating  the  solution  to  about  50°  (it  must  on  no 
account  be  heated  to  boiling,  else  the  reduced  metal  will 
flake  off  from  the  platinum,  and  cannot  be  determined),  the 
burner  shown  in  Fig.  54  is  used.  The  tube  of  a  Bunsen 
burner  may  also  be  unscrewed,  and  the  luminous  jet  issuing 
from  the  opening  at  the  bottom,  reduced  to  a  few  millimetres 
in  height,  used  to  heat  the  solution.  The  distance  of  the 


FIG.  54. 

dish  from  the  burner  must  be  about  15  cm.  To  insure 
uniform  distribution  of  the  reduced  metal  on  the  dish,  it 
must  be  uniformly  heated.  This  is  most  simply  accomplished 
by  placing  under  the  dish  a  piece  of  thin  asbestos  paper  cut 
through  at  the  points  of  contact  of  the  dish  with  the  contact 
wires  of  the  standard.  The  use  of  asbestos  paper  also  dimin- 
ishes the  danger  of  boiling. 

When  the  current  acts  for  a  long  time,  it  is  impossible  to 
prevent  some  evaporation  of  the  solution,  whereby  a  part  of 
the  reduced  metal  is  exposed  to  the  action  of  water-vapor 
and  air.  To  prevent  the  oxidation  of  metal  laid  bare  by 
•evaporation,  a  little  water  is  poured,  from  time  to  time,  on 


THE   PROCESS    OF   ANALYSIS.  77 

the  glass  cover  of  the  dish,  so  that  the  metal  remains  always 
covered  by  the  solution. 

After  precipitation  is  complete,  the  solution  remaining  in 
the  dish  is  poured  into  a  beaker,  with  care  to  avoid  loss,  the 
dish  washed  three  times  with  about  5  cc.  of  cold  water,  and 
then  three  times  with  pure  absolute  alcohol.  The  dish  is 
dried  some  five  minutes  in  an  air-bath  at  70°-90°  C.,  allowed 
to  cool  thoroughly  in  a  desiccator,  and  weighed. 


78  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


DETERMINATION  OF  THE  METALS, 


IRON.* 


IF  the  solution  of  a  ferrous  saltf  is  treated  with  potassium 
or  ammonium  oxalate,  there  is  produced  an  intensely  yellowish 
red  precipitate  of  ferrous  oxalate,  soluble  in  excess  of  the  re- 
agent to  a  yellowish  red  solution  of  the  double  salt. 


Fe2(C204)3  3KA04,*r  Fe2(C2O4)3, 

The  above-named  oxalates  do  not  precipitate  ferric  salts  ; 
but,  if  added  in  sufficient  quantity,  a  solution  of  the  double 
ferric  salt  is  produced  having  a  more  or  less  green  color.  If 
this  solution  is  submitted  to  electrolysis,  there  is  first  produced 
the  double  ferrous  salt,  which  is  then  decomposed,  with  separa- 
tion of  metallic  iron  ;  the  green  liquid  therefore  becomes  first 
red,  and  then  colorless.  Because  of  this  action,  the  determina- 
tions of  iron  is  more  rapidly  performed  in  solutions  of  ferrous 
than  of  ferric  salts.  Potassium  iron  oxalate  is  not  adapted  to 
electrolysis,  because  the  potassium  carbonate  which  is  pro- 
duced precipitates  iron  carbonate,  and  thus  complete  reduc- 
tion is  prevented.  The  electrolysis  of  the  ammonium  double 
salt,  when  ammonium  oxalate  is  in  sufficient  excess,  is  com- 

*  Author's  method. 

f  As  stated  ou  p.  74,  sulphates  are  best  adapted  to  this  treatment,  chlo- 
rides less  so,  while  nitrates  must  be  avoided. 


DETERMINATION   OF   THE   METALS.  79 

plete,  with  no  separation  of  an  iron  compound.  If  the  solu- 
tion contains  free  hydrochloric  acid,  it  is  best  to  remove  it  by 
evaporation  on  the  water-bath. 

Free  sulphuric  acid  may  be  neutralized  with  ammonia, 
since  the  ammonium  sulphate  thus  produced  only  increases 
the  conductivity  of  the  solution.  Nitrates  are  converted  into 
sulphates  or  chlorides  by  evaporation  with  sulphuric  or  hydro- 
chloric acid ;  in  the  latter  case,  repeated. 

The  determination  is  conducted  as  follows:  Assuming 
that  1  gm.  of  iron  may  be  present  in  the  solution  to  be  elec- 
trolyzed,  some  6  gm.  of  ammonium  oxalate  are  dissolved  by 
heat  in  as  little  water  as  possible,  and  the  iron  solution  grad- 
ually added,  with  constant  agitation.*  The  solution  is  then 
diluted  to  150-1Y5  cc.,  warmed  (see  p.  Y6),  and  electrolyzed. 
The  decomposition  is  begun  with  a  current  of  10  or  12  cc. 
oxyhydrogen  gas  per  minute,  which  is  increased,  toward  the 
close  of  the  operation,  to  15-20  cc.  In  case  a  red  flocculent 
precipitate  of  ferrous  oxalate  appears,  oxalic  acid  is  added 
-drop  by  drop  until  it  dissolves. 

The  end  of  the  reaction  is  determined  by  taking  out  a 
small  portion  of  the  colorless  solution  with  a  capillary  tube, 
acidifying  strongly  with  hydrochloric  acid,  and  testing  with 
potassium  sulphocyanate.  When  the  reaction  is  ended,  the 
positive  electrode  is  removed  from  the  solution,  which  is 
poured  off,  and  the  dish  washed  three  times  with  cold  water 
(about  5  cc.  each  time),  and  three  times  with  absolute 
alcohol,  dried  a  few  moments  in  the  air-bath  at  a  temperature 
of  70°  to  90°,  and  weighed  after  cooling. 

The   separated   iron   has  a   steel-gray  color   and   brilliant 

*  It  is  not  desirable  to  add  ammonium  oxalate  solution  to  a  ferrous 
solution,  as  difficultly  soluble  ferrous  oxalate  separates,  and  can  be  dis- 
solved to  the  double  salt  only  by  long  beating.  With  a  ferric  solution  this 
precaution  is  unnecessary. 


80  QUANTITATIVE   ANALYSIS   BY   ELECTKOLYSIS. 

lustre,  is  firmly  attached  to.  the  dish,  and  can  be  preserved  in 
the  air  without  oxidation  for  a  full  day. 

According  to  the  experiments  in  the  Munich  laboratory, 
the  solution  containing  8  gm.  ammonium  oxalate,  and  suffi- 
ciently diluted,  is  warmed  and  electrolyzed  with  a  current  of 
N  .  D100  =  0.5  —  1  ampere  (see  p.  63). 

F.  Riidorff  has  determined  the  conditions  under  which 
iron  may  be  precipitated  from  the  ammonium  double  oxalate 
solution  by  the  use  of  Meidinger  cells.  The  solution  to  be 
electrolyzed  should  contain  not  more  than  0.3  gm.  iron.  If 
free  acid  is  present  the  solution  is  neutralized  with  ammonia 
treated  with  60  cc.  of  a  solution  of  ammonium  oxalate  satu- 
rated at  the  ordinary  temperature,  diluted  to  about  120  cc., 
and  electrolyzed  with  a  battery  of  6  or  8  cells.*  Complete 
precipitation  requires  about  14  hours,  against  some  4  hours  by 
the  author's  process. 

According  to  experiments  conducted  in  the  Aachen  labo- 
ratory by  A.  Brand,  iron  may  be  precipitated  from  a  solution 
containing  sodium  pyrophosphate  in  excess.  Ferric  salts 
yield  a  white  precipitate  with  this  reagent,  and  give  with  an 
excess  a  colorless,  ferrous  salts  a  clear  green,  solution.  The 
solution  is  made  alkaline  with  ammonium  carbonate,  and 
electrolyzed  with  a  current  of  20-30  cc.  per  minute.  The 
end  of  the  reaction  is  determined  by  testing  with  ammonium 
sulphide.  The  precipitate  must  be  washed  without  breaking 
the  current,  else  iron  will  go  in to^ solution. 

Edgar  F.  Smith  precipitates  iron  from  a  solution  of  ammo- 
nium citrate  and  a  few  drops  citric  acid  by  a  current  of  6-15 
cc.  OH  gas.  The  author's  experiments  in  earlier  years  on  the 

*  Rudorff  uses  as  negative  electrode  a  crucible-shaped  platinum  dish, 
60  mm.  high,  75  min.  diameter  at  top,  of  about  40  gin.  weight,  and  170  cc. 
capacity,  and  as  positive  electrode  a  stout  platinum  wire  partially  wound 
into  a  spiral  form.  (See  Fig.  43.) 


DETERMINATION    OF   THE   METALS.  81 

separation  of  iron  from  other  metals  in  citric  and  tartaric  acid 
solutions  indicated  that  in  the  presence  of  fixed  organic  acids 
the  precipitated  metals  always  contain  carbon. 

COBALT. 

Cobalt  can  be  very  easily  precipitated  from  a  solution  of 
cobalt  ammonium  oxalate  containing  an  excess  of  ammonium 
oxalate.  The  metal  separates  rapidly  at  the  negative  electrode 
in  a  compact  adherent  coating  showing  its  characteristic  metal- 
lic properties.  The  process  resembles  the  preceding.  5  or  6 
grn.  ammonium  oxalate  are  dissolved  by  heating  in  the  solu- 
tion of  about  25  cc. ;  it  is  diluted  to  150  or  175  cc..  and  sub- 
mitted warm  to  electrolysis.  The  operation  is  performed  as 
in  the  electrolytic  determination  of  iron. 

Another  method  is  to  add  to  the  cobalt  solution  15-20  cc. 
ammonium  sulphate  solution  (300  gm.  (NH4)2SO4  to  the  litre), 
then  40  cc.  ammonia  of  sp.  gr.  0.96  (if  more  than  0.5  gm. 
cobalt  are  present,  50-60  cc.  NH3),  dilute  with  water  to  150- 
170  cc.,  and  electrolyze  with  a  current  of  about  5  cc.  oxyhv- 
drogen  gas  (N  .  D100  =  0.7  amperes — maximum  —  p.  54)  at  the 
ordinary  temperature  (Fresenius  and  Bergmann).  The  pres- 
ence of  chlorides  and  nitrates  is  unfavorable  to  the  reduction. 

Fixed  organic  acids  (citric,  tartaric,  etc.)  and  magnesium 
compounds  also  act  injuriously. 

F.  Riidorff  recommends  the  following  proportions  for  the 
preceding  method  when  Meidinger  cells  are  used.  The  solu- 
tion, which  may  contain  0.1-0.3  gm.  cobalt,  is  treated  with 
25  cc.  of  a  saturated  solution  of  ammonium  sulphate,  and  an 
equal  volume  of  ammonia,  sp.  gr.  0.91,  and  electrolyzed  after 
dilution  to  100  cc.  by  the  use  of  a  battery  of  3-6  cells.  The 
reaction  requires  12-14  hours;  ammonium  sulphide  is  used  to 
test  its  completion. 


82  QUANTITATIVE   AKALYSIS   BY   ELECTROLYSIS. 

According  to  A.  Brand,  cobalt,  like  iron,  may  be  separated 
from  a  solution  of  the  double  pyrophosphate.  Sodium  pyro- 
pbosphate  is  added  until  the  cobaltous  pyrophosphate  which 
separates  is  completely  redissolved,  then  ammonium  carbonate 
in  slight  excess,  and  the  reduction  effected  by  a  current  of 
about  3  cc.  OH  gas.  To  throw  down  the  last  traces  it  is 
desirable  to  use  a  current  of  about  15  cc.  The  metal  must  be 
washed  while  the  current  is  unbroken. 

NICKEL. 

Nickel  may  be  reduced  under  conditions  similar  to  those 
requisite  for  cobalt;  the  metal  is  precipitated  from  the  solu- 
tion of  the  double  oxalate  containing  ammonium  oxalate  in 
excess,  by  the  action  of  a  similar  current,  as  a  thick  bright 
coating  on  the  negative  electrode.  The  end  of  the  reaction  is 
ascertained  by  testing  with  ammonium  sulphide  or  potassium 
sulphocarbonate,  and  the  precipitate  is  treated  as  previously 
directed. 

According  to  Fresenius  and  Bergmann,  nickel,  like  cobalt, 
may  be  precipitated  completely  from  a  solution  treated  with 
ammonium  sulphate  and  ammonia  (see  Cobalt). 

According  to  A.  Brand,  nickel  also  may  be  precipitated 
from  a  double  pyrophosphate  solution.  The  process  is  the 
same  as  with  cobalt.  A  current  of  2-3  cc.  OH  gas  per  min- 
ute will  precipitate  0.2-0.3  gm.  nickel  within  24  hours. 
More  rapid  precipitation  ensues  by  the  use  of  a  current  of 
about  20  cc.  OH  gas. 

F.  Itiidorff  gives  the  following  directions  for  this  process. 
The  nickel  solution  is  treated  with  25  cc.  of  a  saturated  solu- 
tion of  sodium  pyrophosphate,  then  with  an  equal  volume  of 
ammonia,  sp.  gr.  0.91,  and  electrolyzed  with  the  current  from 
4  to  6  Meidinger  cells. 


DETERMINATION   OF   THE   METALS.  83 

ZINC. 

Zinc  separates  from  the  solution  of  the  ammonium  double 
oxalate  as  readily  and  quickly  as  the  preceding  metals. 

The  reduced  metal  has  a  bluish  white  color,  and  adheres 
well  to  the  negative  electrode,  though  not  so  firmly  as  iron, 
cobalt,  and  nickel.  The  solution  is  not  heated;  a  current  of 
5-6  cc.  oxy hydrogen  gas  is  used;  the  end  of  the  reaction  is 
determined  by  potassium  ferrocyanide  (with  heat).  The 
separated  metal,  which  is  to  be  washed  with  water  and  alco- 
hol, as  previously  directed,  adheres  so  closely,  after  drying,  to 
the  dish,  that  it  is  with  difficulty  dissolved  by  heating  with 
acid ;  ordinarily  a  dark  coating  of  platinum  black  remains, 
which  can  only  be  removed  by  igniting  the  dish  and  treating 
again  with  acid.  For  this  reason,  it  is  desirable,  before 
weighing  the  dish,  to  precipitate  on  it  a  thin  coating  of 
copper,  tin,  or,  better,  silver. 

To  coat  the  platinum  dish  with  copper,  a  solution  of  copper 
sulphate  acidified  with  nitric  acid  is  decomposed  by  a  current 
of  2-3  C2.  OH  gas  for  a  few  hours.  The  copper  precipitate 
is  washed  with  water  and  alcohol,  and  the  dish  dried  in  the 
air-bath  for  a  short  time  at  90°-100°.  A  coating  of  tin  is 
obtained  from  a  double  oxalate  solution  (see  Tin)  and  one  of 
silver  from  a  potassium  cyanide  solution  (see  Silver). 

According  to  v.  Miller  and  Kiliani  4  gm.  potassium  oxa- 
late and  3  gm.  potassium  sulphate*  are  dissolved  in  water, 
the  neutralized  zinc  solution  (sulphate  or  nitrate  containing 
not  more  than  0.3  gm.  Zn)  carefully  added,  and  electrolysis 
effected  without  heat  by  a  current  of  N" .  D100  ==  0.3-05  amperes 
{see  p.  63).  The  reaction  is  complete  in  2  to  3  hours. 

*  The  reduction  of  zinc  from  solution  of  zinc-potassium  oxalate  is  often 
•credited  to  Reinhardt  and  Ihle.  The  author  described  this  method  in 
Fehling's  Dictionary  (on  which  he  was  a  collaborator)  before  the  paper  of 
the  above-named  workers  appeared  in  the  "  Journ.  f.  prakt.  Chemie." 


84  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

Beilstein  and  Jawein  precipitate  zinc  from  a  solution  of 
potassium  zinc  cyanide.  The  solution  is  treated  with  sodium 
hydroxide  till  a  precipitate  is  formed,  and  potassium  cyanide 
is  added  till  it  is  dissolved.  Four  Bunsen  cells  are  required 
for  the  reduction.  As  the  strong  current  heats  the  solution, 
the  authors  recommend  that  it  be  cooled. 

Parodi  and  Mascazzini  determine  zinc,  by  electrolysis,  in 
a  solution  of  the  sulphate,  to  which  has  been  added  sodium 
acetate  and  citric  acid  to  acid  reaction.  The  solution,  diluted 
to  about  175  cc.,  is  exposed  to  the  action  of  a  current  of  4  or 
5  cc.  oxyhydrogen  gas  per  minute. 

When  the  precipitation  is  complete,  the  solution  must  be 
removed  by  a  siphon  without  interrupting  the  current. 

Riche  reduces  zinc  from  a  solution  of  the  acetate  with 
excess  of  ammonium  acetate,  obtained  by  supersaturatioii 
with  ammonia,  and  acidifying  with  acetic  acid. 

F.  Riidorff  gives  the  following  directions  for  the  foregoing 
process.  To  the  solution,  which  must  contain  not  more  than. 
0.25  gm.  zinc,  is  added  20  cc.  of  a  25$  solution  of  sodium 
acetate,  and  about  3  drops  of  diluted  acetic  acid  (50$).  It  is 
diluted  with  water  until  in  the  copper-coated  platinum  disk 
1—2  cm.  of  the  copper  film  are  left  uncovered.  The  elec- 
trolysis is  conducted  with  a  battery  of  5  or  6  Meidinger  cells. 
To  prevent  solution  of  the  zinc  the  dish  must  be  washed  as 
rapidly  as  possible.  The  temperature  in  the  air-bath  in  which 
the  dish  is  dried  must  not  exceed  60°.  This  method  is  adapted 
only  to  solutions  which  contain  zinc  as  sulphate. 

A.  Brand  separates  zinc  from  a  solution  containing  sodium 
phosphate  in  excess  (see  Iron).  The  solution  in  which  the 
double  salt  has  been  produced  is  made  strongly  alkaline  with 
ammonium  carbonate,  and  the  electrolysis  conducted  with  a 
current  of  5-10  cc.  OH  gas  .per  minute.  To  completely  pre- 
cipitate the  zinc  the  current  strength  is  increased  at  the  close 


DETERMINATION    OF   THE   METALS.  85 

to  15-20  cc.  The  metal  must  be  washed  without  interrupting 
the  current. 

G.  Wartmann  has  experimented  in  the  Aachen  laboratory 
on  the  separation  of  metals  as  amalgams.*  Zinc  may  be 
separated  as  amalgam  either  from  solution  of  ammonium-zinc 
oxalate  (p.  83)  or  from  ammoniacal  solution.  A  weighed 
quantity  of  mercuric  chloride  and  3-5  gm.  ammonium  oxa- 
late are  dissolved  in  the  solution  of  the  zinc  salt,  it  is  diluted 
with  water  and  electrolyzed.  A  current  of  6-8  cc.  OH  gas  is 
used  for  a  few  minutes,  it  is  then  reduced  about  one  half,  and 
gradually  increased,  as  the  reaction  progresses,  to  the  original 
strength. 

The  proportion  of  mercury  to  zinc  must  not  be  more  than 
2  or  3  to  1.  The  amalgam,  after  washing  with  water,  alcohol, 
and  ether,  is  dried  to  constant  weight  in  a  desiccator. 

In  the  determination  from  ammoniacal  solution,  tartaric 
acid  and  ammonia  in  excess  are  added.  In  this  case  there 
must  be  present  at  least  three  times  as  much  mercury  as  zinc. 

As  the  determination  of  zinc  from  the  double  oxalate  solu- 
tion and  by  Riche's  method  presents  no  difficulties,  there  is  no 
practical  advantage  in  these  amalgam  methods. 

MANGANESE. 

Manganese  is  one  of  the  metals  which  are  oxidized  by 
the  current  to  the  peroxide,  which  separates  at  the  positive 
electrode.  The  separation  of  the  dioxide  is  complete  from  a 
solution  of  potassium  manganese  oxalate  (it  is  not  complete 
from  a  solution  of  the  ammonium  double  salt),  and  also  from 
solutions  containing  free  nitric  or  sulphuric  acid.  By  the 
former  method  the  solution  of  the  manganese  salt  is  treated 

*  The  determination  of  zinc  as  amalgam  was  described  by  C.  Luckow 
in  1885. 


86  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

with  potassium  oxalate  in  slight  excess,  diluted,  and  precipi- 
tated with  a  current  of  9-12  cc.  oxyhydrogen  gas  per  minute. 
If  the  quantity  is  small,  the  dioxide  adheres  to  the  positive 
electrode  firmly  enough  to  be  converted,  after  washing,  into 
mangano-manganic  oxide  by  ignition  of  the  previously  weighed 
electrode.  With  larger  quantities  it  is  best  to  make  the  plati- 
num dish  the  positive  electrode,  and,  when  the  reaction  is 
ended,*  to  endeavor  to  convert  the  dioxide  into  mangano- 
manganic  oxide  by  ignition,  without  filtration.  If  this  is 
impossible,  the  precipitate  is  filtered  off,  washed  with  hot 
water,  and  converted  either  into  mangano-manganic  oxide  or 
sulphate. 

The  foregoing  method  is  also  available,  as  will  be  described 
later,  for  the  separation  of  manganese  from  many  metals,  and 
the  simultaneous  determination  of  the  latter. 

Manganese  may  be  separated  from  acid  solution  in  the 
presence  of  free  nitric  or  sulphuric  acid. 

In  the  first  case  the  solution  of  manganese  nitrate  (which 
must  not  contain  more  than  0.05  gin.  Mn)  is  acidified  with 
nitric  acid,  the  platinum  dish  is  made  the  positive  electrode 
and  a  platinum  spiral  (p.  68)  placed  in  it  as  negative  electrode, 
the  solution  is  gradually  heated  on  the,  water-bath  to  about  50°, 
and  electrolyzed  with  a  current  of  about  0.03  cc.  OH  gas  per 
minute.  Since  nitric  acid  is  converted  into  ammonia  (p.  3) 
the  solution  is  tested  from  time  to  time  and  kept  acid  by  addi- 
tion of  nitric  acid.  The  coating  of  manganese  dioxide  is  care- 
fully washed  with  water,  and  the  dish  dried  either  in  the  air- 
bath  at  60°  or  in  a  desiccator  over  sulphuric  acid  to  constant 
weight.  The  coating  has  the  composition  MnOa  +  H2O. 

*  Ammonium  sulphide  is  entirely  unsuited  to  show  the  end  of  the 
reaction,  since  oxalic  acid  hinders  the  precipitation  of  the  sulphide.  The 
best  method  is  to  evaporate  a  test  portion  on  platinum-foil,  and  fuse  with 
sodium  carbonate. 


DETERMINATION    OF   THE   METALS.  87 

The  MnOu  is  converted  by  ignition  into  Mn3O4.  This  may 
also  be  weighed. 

If  the  manganese  is  present  as  sulphate,  F.  Hiidorff 
recommends  to  acidify  with  3  drops  sulphuric  acid,  dilute 
with  water  to  100  cc.,  and  electrolyze  with  a  battery  of  2 
Meidinger  cells.  The  maximum  weight  of  manganese  allow- 
able in  the  solution  is  O.Oi  gm.  The  precipitation  is  complete 
in  12-14  hours;  it  is  tested  by  ammonium  sulphide. 

The  manganese  dioxide  is  readily  removed  from  the  dish 
with  very  dilute  sulphuric  acid  to  which  hydrogen  peroxide  is 
added. 

For  the  separation  of  manganese  as  peroxide  from  solution 
of  the  double  pyrophosphate,  A.  Brand  proceeds  as  follows : 
the  solution  is  treated  with  sodium  pyrophosphate  in  abundant 
sufficiency  for  the  formation  of  the  double  salt,  then  with 
ammonia  until  the  precipitate  which  has  been  formed  dis- 
solves. If  not  more  than  0.02  gm.  Mn  to  100  cc.  is  present, 
the  electrolysis  is  conducted  with  a  current  of  0.1  cc.  OH  gas 
per  minute ;  if  more  manganese  is  present  the  action  is  begun 
with  a  current  of  0.01  cc.,  which  toward  the  end  is  increased 
to  0.04  cc.  Brand  recommends  to  wash  the  dioxide  with 
water  only,  as  alcohol  tends  to  cause  the  coating  to  scale  off; 
he  converts  the  MnO.,  into  Mn3O4  by  ignition  over  the  blast- 
lamp.  Brand  states  that  when  the  platinum  dish  is  made  the 
positive  electrode,  0.2  gm.  Mn  can  be  precipitated  from  a  150- 
cc.  solution  as  a  firmly  adherent  coating  of  dioxide. 

ALUMINIUM,  CHROMIUM,   URANIUM,   BERYLLIUM. 

If  a  solution  of  aluminium  ammonium  oxalate  containing 
ammonium  oxalate  in  excess  is  submitted  to  the  action  of  the 
electric  current,  the  ammonium  oxalate  is  changed  into  car- 
bonate and  the  aluminium  separates  as  hydroxide.  When 
the  oxalate  is  decomposed,  the  solution  is  heated  till  it  smells 


88  QUANTITATIVE   ANALYSIS    BY   ELECTROLYSIS. 

only  slightly  of  ammonia,  the  hydroxide  filtered  off,  washed 
with  water,  and  converted,  by  ignition,  into  A12O3. 

Uranium  is  acted  on  in  the  same  way  as  aluminium. 

Chromium  ammonium  oxalate  is  oxidized  by  the  current 
with  formation  of  ammonium  chromate.  To  determine  the 
chromic  acid,  the  ammonium  carbonate  is  decomposed  by 
boiling,  the  solution  acidified  with  acetic  acid,  and  the  chromic 
acid  determined  as  lead  or  barium  chromate. 

When  beryllium  ammonium  oxalate  is  subjected  to  elec- 
trolysis, the  beryllium  is  kept  in  solution  by  the  hydrogen 
ammonium  carbonate  produced,  provided  the  solution  is  cold. 

The  behavior  of  aluminium,  chromium,  uranium,  and 
beryllium  can  be  made  use  of,  as  explained  later,  to  separate 
them  from  each  other,  and  from  all  metals  which  separate 
from  their  double  oxalates,  in  the  metallic  state,  at  the  nega- 
tive electrode. 

COPPER. 

For  the  reduction  of  copper  from  a  solution  containing 
ammonium  oxalate  in  excess,  it  is  possible  to  use  only  a  weak 
current ;  for  otherwise  the  metal  separates  in  a  spongy  condi- 
tion at  the  negative  electrode.  Since  it  is  not  always  possible 
to  obtain  an  adherent  deposit  of  copper  from  the  solution  of 
this  double  salt,  the  author  has  discontinued  this  method,  and 
as  long  ago  as  1888  began  experiments  on  the  determination 
of  this  metal  from  a  solution  of  the  acid  double  oxalate.* 
The  copper  solution  is  treated  with  a  cold  saturated  ammo- 
nium oxalate  solution  and  the  electrolysis  begun  with  a 
current  of  3-4  cc.  OH  gas.  When  the  separation  of  the  cop- 
per is  under  way,  and  the  original  deep-blue  color  of  the  solu- 

*  Ber.  d.  Cb.  Ges.,  21,  2898.  The  author  makes  this  reference  because 
Rudorff •  (Zeit.  f.  angew.  Chem.,  1892)  criticises  the  older  method,  and 
makes  no  mention  of  the  later  procedure. 


DETERMINATION    OF   THE   METALS.  89 

tion  begins  to  fade,  a  cold  saturated  solution  of  oxalic  acid  is 
added.  The  weaker  in  copper  the  solution  appears,  the  more 
abundantly  may  oxalic  acid  be  added ;  altogether  25-30  cc. 
can  be  used.  In  the  analysis  of  substances  poor  in  copper 
the  oxalic  acid  may  be  added  to  the  solution  at  the  beginning, 
but  in  more  concentrated  copper  solutions  the  electrolysis 
must  be  conducted  in  a  solution  as  nearly  neutral  as  possible, 
lest  difficultly  soluble  copper  oxalate  be  precipitated  by  the 
free  oxalic  acid. 

When  the  decomposition  is  complete,  the  solution  is  poured 
off,  the  dish  with  its  coating  washed  first  with  water,  then 
with  alcohol,  dried  in  the  air-bath,  and  weighed  after  cooling. 

The  copper  precipitate  has  a  lively  red  color,  is  closely 
adherent,  and  has  little  resemblance  to  the  precipitate  from 
nitric  acid  solution  (see  below).  The  greatest  advantage  of 
this  method  is  the  rapidity  with  which  it  may  be  conducted. 
If  the  solution  is  warmed  to  40°-50°,  and  this  temperature 
maintained  during  the  reaction,  it  is  possible  to  separate  2  gms. 
copper  in  3  to  4  hours,  as  a  beautiful  adherent  precipitate.  No 
trace  of  copper  can  be  detected  in  the  supernatant  liquid. 

Copper  may  also  be  precipitated  completely,  as  Luckow 
has  shown,  from  nitric  acid  solution;  the  separation  is  as  easy 
and  even  surer  from  sulphuric  acid  solution.  Precipitation 
from  acid  solutions  has  the  drawback,  however,  that  the  cop- 
per must  be  washed  without  interrupting  the  current;  this 
Increases  the  bulk  of  the  solution,  and  renders  more  difficult 
the  determination  of  other  metals  in  it. 

The  reduction  of  copper  from  nitric  acid  solution  requires 
the  use  of  a  definite  amount  of  nitric  acid,  and  the  absence  of 
chlorides.  To  about  200  cc.  of  solution  containing  the  copper 
as  sulphate  is  added  20  cc.  nitric  acid,*  sp.  gr.  1.21,  and  the 
*  This  large  excess  of  nitric  acid  is  necessary  only  when  copper  is  to  be 
separated  from  other  metals.  If  no  other  metal  is  present,  1-2  cc.  nitric 
acid  suffices. 


90  QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

electrolysis  conducted  with  a  current  of  3-4  cc.  OH  gas- 
(N  .  D100  =  1  ampere,  when  no  other  metal  than  copper  is 
present,  otherwise  0.5  ampere,  see  p.  63). 

Chlorides  must  not  he  present.  If  antimony,  arsenic,  or 
bismuth  be  present  traces  of  these  metals  are  thrown  down 
with  the  copper. 

F.  liiidorff  gives  the  following  directions  for  precipitating 
copper  by  the  use  of  a  battery  of  2  to  6  Meidinger  cells : 
The  copper  solution,  neutralized  with  ammonia  or  sodium 
carbonate,  is  treated  with  about  5  drops  nitric  acid,  sp.  gr. 
1.20,  and  diluted  with  water  to  100  cc.  In  presence  of  0.1- 
0.4  gm.  copper  (in  the  form  of  nitrate  or  sulphate)  the  precipi- 
tation is  complete  in  12-14  hours.  The  close  of  the  reaction 
is  ascertained  by  testing  with  hydrogen  sulphide  solution. 

The  presence  of  chlorides  causes  the  precipitate  to  be 
flocculent.  To  avert  this  action  and  secure  an  adherent  pre- 
cipitate, 2-3  gm.  ammonium  nitrateand  20 cc.  ammonia  sp.gr. 
0.96  are  added,  the  solution  diluted  to  100  cc.,  and  electrolyzed 
with  a  battery  of  4-6  Meidinger  cells.  At  the  close  of  the 
reduction  the  solution  is  acidified  with  dilute  acetic  acid,  the 
dish  filled  with  water  to  overflowing,  emptied,  shaken  to  remove 
the  last  drops  of  water,  and  dried  at  100°  in  the  air-bath. 

The  determination  of  copper  from  ammoniacal  solution, 
according  to  F.  RiidorfL,  is  conducted  at  the  Munich  laboratory 
as  follows :  Ammonia  is  added  in  slight  excess  until  the  pre- 
cipitate at  first  appearing  is  redissolved.  Then  20-25  cc. 
ammonia,  sp.  gr.  0.96,  is  added  in  case  not  more  than  0.5  gm. 
Cu  is  present.* 

In  this  solution  3-4  gm.  ammonium  nitrate  is  dissolved, 
and  the  electrolysis  conducted  with  a  current  of  N  .  D100  =  2 
amperes.  The  precipitate  must  be  washed  without  interrupt- 
ing the  current. 

*  If  as  much  as  1  gm.  Cu  is  present,  30-35  cc.  ammonia  must  be  added. 


DETERMINATION   OF   THE   METALS.  91 

A  solution  containing  free  nitric  acid  may  also  be  used 
for  the  separation  of  copper  from  such  metals  as  are  not 
reduced  in  the  presence  of  this  acid,  or  separate  as  peroxides 
at  the  positive  electrode  (e.g.,  cobalt,  nickel,  zinc,  cadmium, 
iron  ;  manganese,  lead).  It  is,  however,  to  be  remembered 
that  nitric  acid  is  gradually  converted  into  ammonia;  and  it 
is  therefore  necessary,  if  the  action  is  long  continued,  to  add 
citric  acid  from  time  to  time.  Copper  may  be  precipitated 
from  ammonium  oxalate,  or  nitric  acid,  solution,  in  the 
presence  of  small  quantities  of  antimony  arid  arsenic.  If, 
however,  any  considerable  amount  of  these  metals  is  present, 
the  antimony  and  arsenic  are  deposited  on  the  copper  after 
the  current  has  passed  for  a  long  time,  so  that  the  negative 
electrode  is  more  or  less  darkened.  To  determine  the  copper, 
in  such  cases,  the  dry  electrode  is  ignited  for  a  short  time, 
thus  oxidizing  the  copper,  and  volatilizing  the  arsenic  and 
antimony.  The  oxide  is  then  dissolved  in  nitric  acid,  and  the 
electrolysis  repeated  (Mansfeld  Smel  ting-works). 

BISMUTH. 

The  electrolysis  of  bismuth  presents  a  difficulty,  inasmuch 
as  it  is  not  possible  to  precipitate  considerable  quantities  of 
the  metal  as  a  close  adherent  coating  on  platinum. 

Bismuth  is  always  obtained  in  the  same  condition,  whether 
it  is  precipitated  from  acid,  ammonium  oxalate,  or  potassium 
tartrate  solution.  If  care  is  taken  to  have  as  large  a  surface 
as  possible,  and  the  dish  serving  as  negative  electrode  is  filled 
almost  to  the  brim,  the  precipitate  may  be  successfully  washed 
with  water  and  alcohol  if  the  quantity  of  bismuth  is  not  great. 
If,  however,  metallic  particles  separate  from  the  dish,  they 
must  be  collected  on  a  filter  and  separately  weighed. 

The  electrolysis  is  performed  by  adding  to  the  solution  an 
excess  of  ammonium  oxalate,  and  reducing  in  the  cold,  with  a 


92  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

current  of  about  0.02  cc.  oxyhydrogen  gas  per  minute.  During 
the  decomposition,  a  coating  of  peroxide  is  formed  on  the 
positive  electrode,  but  gradually  disappears.  In  order  to  pre- 
vent the  oxidation  of  the  separated  metal,  it  is  necessary  to 
remove  the  last  trace  of  water  by  repeated  washing  with 
absolute  alcohol. 

[Smith  and  Knerr*  find  that  bismuth  completely  and 
rapidly  separates  from  a  sulphate  solution  containing  free 
sulphuric  acid.  They  add  1  cc.  H2SO4  to  25  cc.  solution,  and 
use  currents  producing  from  1  to  4  cc.  oxyhydrogen  gas  per 
minute. — Trans."] 

According  to  A.  Brand,  good  results  are  obtained  by  treat- 
ing the  dilute  acid  bismuth  solution  with  four  or  five  times 
the  quantity  of  sodium  pyrophosphate  needed  to  produce  the 
double  salt,  making  it  barely  alkaline  with  ammonium  carbon- 
ate, and  dissolving  in  this  solution  3-5  gm.  ammonium  oxalate. 
The  solution  is  diluted  to  about  200  cc.,  and  submitted  to  the 
action  of  a  current  of  0.1  to  0.5  cc.  OH  gas,  which  is  gradually 
increased  to  2-3  cc.  Beginning  with  a  current  of  0.5  cc., 
0.25  gm.  bismuth  will  be  reduced  within  12  hours. 

If  a  coating  of  bismuth  peroxide  appears  on  the  positive 
electrode  it  may  be  removed  (but  only  toward  the  end  of  the 
reaction)  by  a  few  drops  of  oxalic  acid.  The  end  of  the  reac- 
tion is  determined  by  testing  with  hydrogen  sulphide  solution. 
Instead  of  weighing  the  metal,  which  is  subject  to  slight  oxi- 
dation, Brand  recommends  its  conversion  into  oxide  (BiaO8) 
by  solution  in  nitric  acid  and  ignition  of  the  nitrate  after 
evaporation  to  dryness. 

Rudorfl  proposes  the  following  combined  method  for  the 
determination  of  bismuth.  In  case  there  is  not  more  than  0.1 
gm.  of  the  metal  in  the  solution,  the  weak  nitric  acid  solution 

*Am.  Ch.  J.,  8,  207. 


DETERMINATION    OF   THE    METALS.  93 

is  treated  with  sodium  pyrophosphate  until  the  resulting 
precipitate  is  redissolved,  then  there  is  added  20  cc.  of  a 
saturated  solution  of  potassium  oxalate  and  an  equal  volume 
of  potassium  sulphate.  The  solution,  diluted  to  120  cc.,  is 
decomposed  by  the  current  from  4  Meidinger  cells.  The 
reduction  requires  at  least  20  hours.  The  bismuth  obtained, 
after  washing,  is  dried  in  the  air-bath  at  60°. 

G.  Vortrnann  separates  bismuth  as  amalgam  (see  Zinc,  p.  85). 
The  bismuth  compound,  dissolved  in  as  little  hydrochloric 
acid  as  possible,  is  treated  with  a  weighed  quantity  of  mer- 
curic chloride,  and  potassium  iodide  added,  until  the  precipi- 
tate produced  by  this  reagent  is  redissolved.  After  diluting 
with  water,  the  bismuth  amalgam  is  precipitated  as  in  the 
corresponding  case  with  zinc.  During  electrolysis  iodine 
separates,  mixed  with  gas  bubbles,  on  the  surface  of  the  solu- 
tion. When  the  reaction  is  ended,  which  is  determined  by 
testing  with  ammonia  arid  ammonium  sulphide,  concentrated 
sodium  hydroxide  solution  is  added,  without  interrupting  the 
current,  to  dissolve  the  iodine,  which  action  is  hastened  by 
stirring  the  solution  with  the  positive  electrode.  If  the  current 
is  now  increased  and  allowed  to  pass  for  an  hour,  the  precipita- 
tion is  complete.  This  is  proved  by  adding  sodium  sulphite 
and  ammonium  sulphide  to  a  test  portion  of  the  solution. 
The  amalgam  is  treated  as  directed  under  Zinc. 

Addition  of  potassium  iodide  may  be  avoided  by  precipita- 
ting the  amalgam  from  a  hydrochloric  acid  solution  treated  with 
alcohol.  The  bismuth  compound  and  the  weighed  quantity 
of  mercuric  chloride  are  dissolved  in  hydrochloric  acid,  and 
about  50  cc.  96$  alcohol  added.  The  solution  is  gradually 
diluted  with  water  until  its  surface  reaches  to  within  1  cm. 
of  the  edge  of  the  dish,  and  then  electrolyzed  as  usual. 

G.  Vortmann  especially  recommends  the  latter  method  for 
the  determination  of  considerable  quantities  of  bismuth.  The 


94  QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

largest  quantity  of  bismuth  determined  in  the  analyses  quoted 
by. him  was  0.833  gm. 

CADMIUM. 

Cadmium  may  be  completely  precipitated  from  solutions 
of  either  the  ammonium  or  potassium  double  oxalate  ;  the 
latter  seems  to  be  preferable.  The  solution  is  treated  with 
an  excess  of  potassium  or  ammonium  oxalate,  diluted  to  about 
175  cc .,  warmed,  and  electrolyzed  by  a  current  of  about  0.05 
cc.  oxyhydrogen  gas  per  minute.  Care  must  be  taken  that 
the  volume  of  the  solution  is  not  reduced  by  evaporation  ; 
water  is  added  from  time  to  time  in  sufficient  quantity  to  keep 
the  separated  metal  covered.  The  precipitation  of  0.2  gm. 
cadmium  occupies  4-5  hours  ;  the  end  of  the  reaction  is  shown 
by  testing  with  hydrogen  sulphide. 

Other  methods  have  been  proposed  for  the  determination 
of  cadmium.  Smith  and  Luckow  recommend  its  precipita- 
tion from  the  solution  of  the  chloride,  sulphate,  or  nitrate, 
after  saturation  with  sodium  acetate.  Eliasberg,  who  tested 
the  method  in  the  laboratory  of  the  author,  found  that  the 
reduction  took  place  readily  when  the  solution  of  about  100 
cc.  volume  was  treated  with  about  3  gm.  sodium  acetate  and 
a  few  drops  of  acetic  acid,  heated,  and  submitted  to  the  action 
of  a  current  of  about  0.6  cc.  oxyhydrogen  gas  per  minute. 

In  the  laboratory  of  the  Munich  high  school  the  foregoing 
method  is  practised  as  follows :  The  solution,  neutralized  if 
necessary,  containing  not  more  than  0.5  gm.  cadmium,  is 
treated  with  3  gm.  sodium  acetate,  and  made  weakly  acid  with 
acetic  acid.  The  solution  is  warmed  to  45°,  and  decomposed 
with  a  current  of  JST  .  D100  =  0.02  —  0.07  ampere  (p.  63). 
The  metal  is  washed  without  interrupting  the  current,  and 
quickly  dried  at  100°. 

According  to  Beilstein  and  Jawein,  cadmium  may  be  de- 


DETERMINATION   OF   THE   METALS.  95 

termined  in  the  same  way  as  zinc.  If  the  solution  contains 
free  acid,  it  is  neutralized  with  potassium  hydroxide,  and 
potassium  cyanide  added  till  the  solution  becomes  clear.  The 
authors  use  a  battery  of  three  Bunsen  cells  (they  do  not  give 
the  strength  of  the  current),  and  dilute  the  solution,  so  that 
0.2  gm.  of  cadmium  are  contained  in  75  cc.  The  dish  in 
which  the  reaction  takes  place  is  cooled  during  the  process. 

According  to  F.  Riidorff,  the  foregoing  process  gives  good 
results  when  conducted  as  follows :  The  solution,  as  nearly 
neutral  as  possible,  containing  not  more  than  0.4  gm.  cad- 
mium, is  treated  with  potassium  cyanide  until  the  solution  be- 
comes clear,  diluted  to  about  100  cc.,  and  electrolyzed  with 
the  current  from  3-6  Meidinger  cells.  The  metal  is  washed 
with  water  and  alcohol,  and  dried  at  70°-80°. 

A.  Brand  converts  the  cadmium  into  pyrophosphate  by  the 
use  of  sodium  pyrophosphate,  and  dissolves  it  in  a  great  ex- 
cess of  ammonia.  For  the  electrolysis  a  current  of  2-3  cc. 
OH  gas  is  used  for  a  few  seconds,  then  gradually  reduced  to 
0.3-1  cc.  Toward  the  close  of  the  process  the  current 
strength  is  again  raised  to  about  5  cc.  Oil  gas. 

Cadmium  may  be  determined  as  amalgam  by  G.  Yort- 
mann's  process,  like  zinc  (p.  85),  from  either  ammonium 
double  oxalate  or  ammoniacal  solution.  At  least  4  parts 
mercury  must  be  used  to  1  part  cadmium.  If  6  parts  of  mer- 
cury are  present,  the  amalgam  is  still  so  hard  that  it  may  be 
rubbed  with  the  linger  without  loss.  With  8  parts  of  mer- 
cury the  amalgam  is  liquid. 

Separation  from  the  double  oxalate  is  desirable  when  only 
small  quantities  up  to  0.3  gm.  of  cadmium  are  present,  be- 
cause of  the  slight  solubility  of  the  cadmium  salt. 

If  larger  quantities  are  present,  the  solution  containing  the 
cadmium  and  mercury  salts  is  treated  with  about  3  gnutar- 
taric  acid,  then  with  ammonia  in  moderate  excess,  and  sub- 


96  QUANTITATIVE   ANALYSIS    BY   ELKCTKOLYSIS. 

mitted  to  electrolysis.     The  end  of  the  reaction  is  determined 
by  testing  with  ammonium  sulphide. 

LEAD. 

If  &  solution  of  a  lead  salt  containing  an  excess  of  ammo- 
nium oxalate  is  decomposed  by  a  current  of  about  0.2  cc.. 
oxyhydrogen  gas  per  minute,  the  lead  indeed  separates  at  the 
negative  electrode,  adheres  closely,  and  shows  its  character- 
istic metallic  properties ;  but  it  oxidizes  partially  on  washing 
with  water  and  alcohol,  so  that  the  results  are  always  too  high. 
The  precipitation  of  lead  as  amalgam  (see  Zinc)  presents 
some  difficulties,  inasmuch  as  some  lead  peroxide  separates  at 
the  positive  electrode,  and  must  be  dissolved.  According  to 
G.  Yortmanu,  the  aqueous  solution  of  the  lead  salt  containing 
sufficient  mercuric  chloride  to  produce  the  amalgam  is  treated 
with  3-5  gm.  sodium  acetate  and  a  few  cubic  centimetres  of 
concentrated  potassium  nitrate  solution.  The  precipitate  pro- 
duced by  the  latter  reagent  (which  is  added  to  prevent  the 
separation  of  peroxide)  is  dissolved  in  acetic  acid,  and  the 
clear  yellow  solution  diluted  and  electrolyzed.  If  lead  perox- 
ide appears  on  the  positive  electrode  during  the  reaction, 
more  potassium  nitrate  is  added.  The  close  of  the  reaction  is 
determined  by  testing  with  ammonium  sulphide.  As  lead 
amalgam  oxidizes  rather  readily  when  moist,  it  is  quickly 
washed  with  water,  alcohol,  and  ether,  dried  by  the  warmth  of 
the  hand  and  by  blowing  upon  it,  and  finally  in  the  desiccator. 

The  amalgam  may  also  be  separated  from  an  aqueous  solu- 
tion acidified  with  nitric  acid.  However,  as  free  nitric  acid 
favors  the  formation  of  lead  peroxide,  more  frequent  addition 
of  potassium  nitrate  is  necessary,  and  complete  precipitation 
is  thereby  seriously  hindered. 

In  a  solution   containing  free  nitric  acid  lead  is  acted  on. 


DETERMINATION  OF  THE  METALS.         97 

jke  manganese;  it  is  oxidized,  and  separates  as  hjdrated 
peroxide  at  the  positive  electrode.  If  there  is  no  other  metal 
in  the  solution,  it  must  contain  at  least  10  per  cent  free  nitric 
acid,  according  to  Luckow ;  in  the  presence  of  other  metals 
(me.viiry,  copper,  etc.),  the  oxidation  is  complete  even  in 
presence  of  little  nitric  acid. 

Small  quantities  of  lead  peroxide  adhere  closely  enough 
to  the  positive  electrode  to  be  determined  by  weight  after 
washing  and  drying  at  110°.  With  larger  quantities,  the 
platinum  dish  is  made  the  positive  electrode,  and  the  process 
carried  on  as  before. 

A  current  of  a  few  tenths  of  a  cc.  of  oxyhydrogen  gas  per 
minute  suffices,  according  to  Luckow,  for  the  precipitation. 
It  must  be  remembered  that  the  peroxide  must  be  washed 
without  interrupting  the  current  to  avoid  loss. 

J.  Messinger,  who  has  tested  this  method  in  the  Aachen 
laboratory,  recommends  the  addition  of  30  cc.  nitric  acid,  sp, 
gr.  1.38,  to  about  175  cc.  of  solution,  and  the  use  of  a  platinum 
dish  as  positive  electrode  and  of  a  current  of  0.1  cc.  (from  2 
Meidinger  cells)  for  the  electrolysis.  The  peroxide  is  washed 
with  water  and  alcohol,  and  dried  at  120°-130°.  It  is  desir- 
able to  reduce  the  current  at  the  beginning  by  means  of  a 
rheostat,  and  increase  it  to  the  given  strength  after  a  coating 
of  peroxide  has  formed.  The  peroxide  adheres  closely,  and 
may  be  washed  without  loss  even  if  as  much  as  0.2  gm.  are 
present. 

Chlorine  compounds  must  not  be  present  in  the  solution 
for  electrolysis. 

In  the  Munich  laboratory  experiments  have  been  con- 
ducted as  to  the  quantity  of  nitric  acid,  sp.  gr.  1.36,  and  have 
demonstrated  that  this  depends  on  the  temperature  and  the 
current  density  (see  p.  63)  to  be  used.  The  current  density 
depends  in  turn  on  the  condition  of  the  surface  of  the  positive 


fc)8  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

electrode.  If  this  is  very  smooth,  a  current  of  N .  Diao  =  0.05 
is  sufficient,  otherwise  one  of  N  .  D100  =  0.5  is  needed  to  pro- 
duce  an  adherent  precipitate.  When  N  .  D100  =  0,05  ampere, 
2  volume-per-cent  of  nitric  acid  should  be  added  when  the 
solution  is  heated,  and  10  volume-per-cent  at  ordinary  tem- 
perature When  N  .  D100  =  0.5  the  volume-percentages  are, 
respectively,  7  and  20  for  heated  and  cool  solutions. 

Heating  the  solution  to  about  50°  materially  assists  the  sep- 
aration. The  precipitate  may  be  washed  without  loss,  after 
the  current  is  cut  off. 

F.  Riidorff  uses  for  the  determination  of  lead  as  peroxide 
the  fact  observed  by  Luckow,  that  lead  in  the  presence  of 
copper  is  precipitated  as  peroxide  from  a  weak  nitric  acid  so- 
lution. He  proceeds  as  follows:  The  solution,  which  must 
contain  not  over  0.1  gm.  of  metal,  is  treated  with  2.3  cc.  nitric 
acid,  and  10  cc.  copper  nitrate  solution,  containing  1  gm.  On. 
in  100  cc.,  is  diluted  to  100-120  cc.  and  electrolyzed  with  3-4 
Meidiuger  cells.  If  the  platinum  dish  is  placed  as  the  posi- 
tive and  platinum  foil  as  the  negative  electrode,  the  lead  per- 
oxide separates  on  the  former  and  the  copper  on  the  latter. 
The  precipitation  is  completed  in  12  hours. 

This  method  can  evidently  be  used  in  only  very  few  cases, 
inasmuch  as  other  metals  than  lead  are  usually  to  be  deter- 
mined. According  to  experiments  in  the  Munich  laboratory, 
lack  of  nitric  acid  causes  the  separation  of  lead  as  metal,  as 
well  as  peroxide,  and  if  the  electrolysis  is  too  long  continued, 
lead  peroxide  goes  into  solution. 

Various  methods  have  been  proposed  for  the  solution  of 
the  peroxide  after  weighing  :  e.g.,  the  nse  of  nitric  add  in  which 
is  placed  a  rod  of  copper  or  zinc,  of  nitric  acid  and  oxalic  acid 
with  heat,  and  of  nitric  acid  with  addition  of  potassium  nitrite. 


DETERMINATION   OF  THE  METALS. 


99 


THALLIUM. 

This  metal   cannot  be   completely  precipitated  from   an 
ammonium  oxalate  solution. 

The  properties  of  thallium  are  similar  to  those  of  lead ; 
its  determination  therefore  requires  special  consideration. 

G.  Neumann,  in  connection  with  a  research  on  certain 
double  salts  of  thallium  in  the  Aachen  laboratory,  has  also 
investigated  the  quantitative  determination  of  the  metal.  As 
his  method  is  of  value  in  the  investigation  of  thallium  coin- 
pounds,  it  is  here  described.  The 
process  is  based  on  precipitation 
of  the  thallium  as  metal,  and 
determination  of  the  volume  of 
hydrogen  set  free  by  its  solution 
in  hydrochloric  acid. 

The  apparatus  shown  in  Fig. 
55  is  used  for  the  process.  K 
is  a  flask  of  about  100  cc.  capac- 
ity, containing  platinum  foil 
electrodes  of  9  sq.  cm.  surface, 
terminating  in  contact  wires 
fused  into  the  glass.  The  thal- 
lium salt  and  about  5  gms. 
ammonium  oxalate  are  dissolved 
in  this  flask,  and  electrolyzed, 
after  dilution,  with  a  current  of 
0.1  ampere.  The  completion  of 
the  reaction  is  ascertained  by 
testing  with  ammonium  sul- 
phide. As  the  ammonium  oxalate  is  converted  into  carbonate 
by  the  current,  and  the  measuring  tube  would  be  insufficient 
to  contain  the  disengaged  carbon  dioxide,  the  solution  remain- 


FIG.  55. 


100        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

ing  in  the  flask  is  removed  after  the  reaction.  This  may 
readily  be  done  by  the  use  of  two  siphons.  Neumann's  au- 
tomatic arrangement  for  this  purpose  is  shown  in  Fig.  56  ;  it 
is  adapted  for  use  in  many  determinations,  and  its  operation 
is  easily  seen  from  the  figure.  The  washing  is  conducted 


FIG.  56. 

without  interrupting  the  current.  To  remove  the  gas  bubbles 
clinging  to  the  electrode  it  is  desirable  to  heat  the  flask  a  short 
time  after  the  washing  is  complete.  The  flask  is  then  con- 
nected to  the  measuring  tube,  and  the  thallium  dissolved  and 
the  hydrogen  collected  and  measured  in  the  usual  way. 

SILVER. 

Silver  forms  with  ammonium  oxalate  a  white  precipitate 
of  silver  oxalate,  which  is  insoluble  in  an  excess  of  the  pre- 


DETERMINATION    OF   THE    METALS.  101 

•cipitant.  This  fact  can  be  utilized  for  the  separation  of  silver 
from  those  metals  which  form  double  salts.  Silver  separates 
from  its  solution  in  potassium  cyanide  with  its  characteristic 
properties  ;  this  solution  is  therefore  well  adapted  to  its  quan- 
titative determination.  (Luckow's  method.)  If  insoluble 
silver  compounds  (silver  oxalate,  silver  chloride)  are  to  be 
analyzed,  they  are  dissolved  in  potassium  cyanide  solution  ;  to 
soluble  compounds  is  added  enough  potassium  cyanide  to 
form  a  double  salt. 

In  order  to  obtain  the  silver  in  dense  deposit,  a  current  is 
used  which  will  produce  1.5  to  2  cc.  oxyhydrogen  gas  per 
minute.  When  the  reduction  is  complete,  the  solution  is 
immediately  poured  off ;  the  silver  is  washed  repeatedly  with 
water,  then  with  alcohol  to  remove  the  water,  dried  in  the  air- 
bath,  and  weighed. 

In  carrying  out  this  method,  according  to  F.  Riidorff,  the 
solution,  which  must  contain  not  over  0.3  gm.  silver,  is  treated 
with  potassium  cyanide  in  slight  excess,  diluted  to  about  100 
cc.,  and  electrolyzed  with- 3  to  6  Meidinger  cells.  The  end  of 
the  reaction  is  ascertained  by  testing  with  hydrogen  sul- 
phide. After  washing  with  water  the  dish  is  dried  in  the  air- 
bath  at  100°. 

J.  KrutWig  treats  the  solution  of  the  silver  salt  with 
ammonia  in  slight  excess,  adds  ammonium  sulphate,  and 
electrolyzes  with  a  current  of  2  cc.  OH  gas  per  minute. 
Toward  the  end  the  current  strength  is  raised  to  about  5  cc. 

In  the  Munich  laboratory  the  following  conditions  have 
been  determined  for  the  preceding  process.  The  solution, 
which  must  not  contain  more  than  0.5  gin.  silver,  is  treated 
with  20  volume-per-cent  of  0.96  sp.  gr.  ammonia  and  5%  am- 
monium sulphate  1 : 10,  warmed,  and  electrolyzed  with  a  cur- 
rent of  N  .  D100  =  0.02  —  0.05  ampere.  The  precipitate  must 


102        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

be  very  thoroughly  washed,  after  the  current  is  cut  off,  to 
completely  remove  the  ammonium  sulphate. 

Fresenius  and  Bergmann  have  found  that  silver  can  also 
be  precipitated  in  a  dense  form  from  a  solution  containing 
nitric  acid.  20  cc.  of  nitric  acid,  sp.  gr.  1.2,  are  added  to  the 
silver  solution  ;  it  is  diluted  to  about  200  cc.,  and  electrolyzed 
with  a  current  of  2-2J  cc.  oxyhydrogen  gas  per  minute. 

According  to  results  in  the  Munich  laboratory,  it  is  desir- 
able to  add  to  the  solution,  which  may  contain  as  much  as  0.4 
gm.  silver,  3  volume-per-cent  of  nitric  acid,  sp.  gr.  1.36,  and 
to  electrolyze  the  heated  solution  with  a  current  of  N  .  Dlon  — 
0.04  —  0.05  ampere  (p.  63).  The  silver  must  be  carefully 
washed  without  interrupting  the  current,  to  prevent  loss.  An 
insufficient  quantity  of  nitric  acid  .may  lead  to  the  production 
of  peroxide. 

MERCURY. 

The  metal  is  readily  precipitated  from  solution  of  mercuric 
salts  treated  with  ammonium  oxalate  in  excess.  A  current  of 
about  2  cc.  OH  gas  is  used.  If  the  mercury  is  present  as 
chloride  in  the  solution  the  electrolytic  action  is  continued 
until  mercurous  chloride  disappears  from  the  positive  electrode. 

Mercury  is  precipitated  from  a  solution  of  a  mercury  salt 
acidified  with  nitric  acid,  by  a  current  of  0.2-0.5  cc.  oxyhydro- 
gen gas,  in  the  form  of  a  mirror,  or  of  small  globules,  on  the 
negative  electrode ;  the  metal  adheres  well  to  the  dish,  and 
can  be  washed  without  loss.  The  washing  must,  however,  be 
done  without  interrupting  the  current,  to  prevent  loss.  The 
metal  washed  with  water  and  absolute  alcohol  is  dried  for  a 
short  time  in  a  desiccator,  over  sulphuric  acid,  and  weighed. 

[Smith  and  Knerr*  use  a  current  of  4  cc.  oxyhydrogen 
gas  per  minute,  and  find  the  precipitation  complete  in  30-45 

*Ain.  Ch.  J.,8,  209. 


DETERMINATION    OF   THE   METALS.  103 

minutes,  the  mercury  forming  a  compact  shining  deposit. — 
TransJ] 

In  the  laboratory  of  the  Munich  high  school  the  following 
conditions  have  been  established  by  experiment.  If  no  other 
metal  than  mercury  is  present,  1-2  volume-per-cent  of  nitric 
acid,  sp.  gr.  136,  is  added ;  otherwise  5  volume-per-cent. 
The  electrolysis  is  conducted  at  ordinary  temperature  with  a 
current  of  N  .  D100  —  1  ampere,  or  JS" .  D100  =  0.5  ampere, 
according  as  mercury  is  the  only  rnetal  present  or  not.  The 
maximum  allowable  quantity  of  mercury  is  2  gm.  The  pre- 
cipitation is  complete  in  2  hours. 

F.  Riidorff  adds  to  the  solution,  which  must  contain  not 
more  than  0.3  gm.  mercury,  about  5  drops  nitric  acid,  sp.  gr. 
1.2,  or  dilute  sulphuric  acid  (1 : 10)  dilutes  to  100  cc.,  and 
electrolyzes  with  the  current  from  2-6  Meidinger  cells.     The 
precipitation  requires  about  14  hours.     When  it  is  complete 
about  10  drops  of -sodium  acetate  are  added,  and  the  precipi- 
tate washed  after  cutting  off  the  current. 

According  to  A.  Brand  the  solution  of  a  mercuric  salt 
(mercurous  salts  must  be  oxidized  to  mercuric)  is  treated  with 
sodium  pyrophosphate  in  slight  excess,  and  the  resulting  pre- 
cipitate dissolved  in  ammonia  or  ammonium  carbonate.  From 
this  solution  a  current  of  2  cc.  electrolytic  gas  per  minute  will 
precipitate  1  gm.  of  mercury  in  5-6  hours. 

According  to  G.  Yortmann,  mercury  also  separates  well 
from  ammoniacal  solution.  The  aqueous  solution  is  treated 
with  tartaric  acid,  and  ammonia  added  in  excess.  F.  Riidorff 
uses  0.5  gm.  tartaric  acid  and  10  cc.  ammonia,  sp.  gr.  0.91, 
and  electrolyzes  the  solution,  diluted  to  100  cc.,  with  the 
current  from  2-6  Meidinger  cells. 

G.  Yortmann  describes  the  determination  of  mercury  by 
the  two  following  methods:  — 

1.  From  sodium  sulphide  solution.     The  aqueous  solution 


104        QUANTITATIVE    ANALYSIS   BY    ELECTROLYSIS. 

of  a  mercuric  salt  is  treated  with  sodium  sulphide  containing 
sodium  hydroxide  until  a  clear  solution  is  obtained.  After 
dilution  with  water  electrolysis  is  conducted  with  a  current  of 
2-3  cc.  electrolytic  gas  per  minute,  which  is  doubled  toward 
the  end  of  the  reaction.  Black  mercuric  sulphide  separates 
at  first  on  the  positive  electrode,  but  disappears  later;  toward 
the  close  the  electrode  becomes  covered  with  a  coating  of 
sulphur. 

2.  From  solution  in  potassium  iodide.  This  method  is 
already  described  under  Bismuth  (determination  as  amalgam, 
p.  93).  The  mercuric  solution  is  treated  with  potassium 
iodide  in  excess,  diluted  and  electrolyzed,  and  the  iodine 
removed  from  the  positive  electrode,  according  to  directions 
there  given. 

Edgar  F.  Smith  precipitates  mercury  from  solution  in 
potassium  cyanide.  The  mercuric  solution,  which  may  con- 
tain about  0.2  gm.  mercury,  is  treated  with  0.25-2  gra.  potas- 
sium cyanide,  diluted  with  water  to  175  cc.,  and  electrolyzed 
with  a  current  of  0.2  cc.  electrolytic  gas.  The  metal  thus 
reduced  must  be  washed  with  water  only,  as  by  washing  with 
alcohol  thin  gray  flakes  are  detached.  Yortmann  makes  the 
same  observation  in  reducing  from  a  solution  of  the  potassium 
double  iodide  made  alkaline  with  sodium  hydroxide. 

Insoluble  mercury  compounds  may  easily  be  analyzed  by 
suspending  them  in  water  acidulated  with  hydrochloric  acid, 
or  a  dilute  solution  of  common  salt  (1 : 10),  and  electrolyzing 
as  usual. 

The  determination  of  mercury  in  cinnabar  is  made  at 
Almaden  by  this  process,  which  originated  with  the  author. 

PLATINUM. 

The  compounds  of  platinum  are  decomposed  with  the 
greatest  ease  by  the  galvanic  current,  with  deposition  of  the 


DETERMINATION   OF   THE   METALS.  105 

metal  on  the  negative  electrode.  If  the  current  from  two 
Bunsen  cells  is  used  for  the  electrolysis,  the  decomposition  is 
so  rapid  that  the  platinum  separates  as  platinum  black,  and 
cannot  be  determined.  If  one  Bunsen  cell  is  used,  the  metal 
separates  in  so  dense  a  form  that  it  cannot  be  separated  from 
hammered  platinum.  It  is  possible,  in  this  way,  gradually  to 
deposit  considerable  quantities  of  platinum  on  the  dish  which 
serves  as  negative  electrode,  without  changing  its  appearance. 

For  the  determination  of  platinum  in  its  salts,  the  solution 
may  be  slightly  acidified  with  hydrochloric  or  sulphuric  acid, 
or  treated  with  ammonium  or  potassium  oxalate,  gently 
warmed,  and  electrolyzed.  The  platinum  separates  in  a  com- 
paratively short  time ;  for  example,  a  solution  of  platinum 
chloride  diluted  to  200  cc.,  containing  0.6  grn.  platinum,  de- 
posited 0.5  gm.  within  five  hours. 

If  the  quantity  of  platinum  is  about  0.4  gm.,  the  solution 
of  the  platinum  salt,  according  to  the  practice  in  the  Munich 
laboratory,  is  treated  with  2  volume-per-cent  of  dilute  sul- 
phuric acid  (1 : 5),  heated,  and  electrolyzed  with  a  current  of 
N  .  D100  —  0.01  —  0.03  ampere ;  the  precipitation  is  complete 
in  about  5  hours. 

Edgar  F.  Smith  adds  to  the  platinum  solution  30  cc.  of 
disodium  phosphate,  5  cc.  phosphoric  acid,  and  dilutes  with 
water  to  150  cc.  With  a  current  of  0.2  to  0.8  cc.  electrolytic 
gas  per  minute,  the  platinum  is  deposited  in  a  coherent  coat- 
ing on  a  platinum  dish  which  has  first  been  coated  with  cop- 
per. 

Iridium  is  not  reduced  from  its  solutions  by  the  current 
of  a  single  Bunsen  cell ;  this  fact  may  be  utilized  for  the 
quantitative  separation  of  platinum  from  indium  (Classen). 


106         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

PALLADIUM. 

Palladium  is  determined  in  the  same  way  as  platinum.  If 
the  current  of  a  single  Bunsen  cell  is  used,  the  palladium  is- 
obtained  in  a  beautiful  metallic  state. 

According  to  Edgar  F.  Smith,  palladium  can  be  deter- 
mined by  the  method  given  under  platinum. 

GOLD. 

Gold  separates  from  the  solution  of  its  compounds  in 
potassium  cyanide,  in  a  beautiful  dense  deposit;  the  process 
is  conducted  as  in  the  determination  of  silver.  Since  the 
gold  can  only  be  dissolved  from  the  platinum  dish  by  aqua 
regia,  the  dish  is  previously  coated  with  a  thin  deposit  of 
silver.  Edgar  F.  Smith  proceeds  in  the  determination  of  gold 
in  a  similar  manner  as  with  platinum. 

ANTIMONY. 

Antimony  is  precipitated  from  hydrochloric  acid  solution, 
but  is  not  adherent.  If  potassium  oxalate  is  added  to  the 
solution  of  the  trichloride,  antimony  is  easily  reduced,  but 
adheres  even  less  closely  than  in  the  other  case.  An  adher- 
ent metallic  deposit  can  be  obtained  by  adding  potassium  tar- 
trate,  but  the  separation  is  then  too  slow. 

The  precipitation  of  antimony  from  the  solutions  of  its 
sulpho-salts  is  complete  and  satisfactory.  If  ammonium  sul- 
phide is  used  to  produce  a  double  salt,  it  must  contain 
neither  free  ammonia  nor  pulysulphides.  Ammonium  sul- 
phydrate,  therefore,  is  convenient  for  the  determination  ;  it 
is  kept  in  small,  tightly  corked  flasks. 

The  solution  for  electrolysis  must  be  cold,  and  the  current 
by  no  means  strong;  it  should  yield  1.5-2  cc.  electrolytic  gas 
per  minute.  When  a  solution  of  antimony  containing  ammo- 


DETERMINATION   OF  THE  METALS.  107 

nium  sulphide  is  eleetrolyzed,  there  is  formed  over  the  metal 
a  coating  of  sulphur  which  cannot  be  washed  off  with  water. 
When  the  metal  is  washed  afterward  with  alcohol,  the  thin 
coating  of  sulphur  can  be  removed  by  rubbing  with  the  fin- 
ger, or  a  handkerchief  moistened  with  alcohol,  without  danger 
of  loss. 

The  use  of  ammonium  sulphide  has  the  disadvantage 
that,  when  several  determinations  are  made  together,  the  odor 
becomes  unbearable.  For  this  reason  the  author  has  made  a 
series  of  experiments  with  potassium  and  sodium  monosul- 
phide  and  hydrosulphide,  the  results  of  which  show  that  the 
precipitation  of  antimony  from  its  double  salts  with  them  pro- 
ceeds satisfactorily.  As  sodium  sulphide  (Na2S)  is  the  one  of 
the  salts  named  which  is  most  desirable  for  facilitating  the 
separation  of  antimony  from  tin  and  arsenic,  the  following 
particulars  relate  exclusively  to  the  use  of  this  salt*  for  the 
determination  of  antimony. 

The  following  formulae  probably  represent  the  reactions 
which  take  place  in  the  electrolysis  of  the  antimony  sulpho- 
salt.  The  current  first  decomposes  water  :  — 


At  the  kathode  :  — 

Sb2S3  +  3JSTa2S  +  6H  =  2Sb  +  6NaHS. 
At  the  anode  :  — 

3O  =  3Na2S2  +  3H2O. 


In  the  separation  of  antimony  from  sodium  sulphide  solu- 
tion it  must  not  be  overlooked,  however,  that  the  presence  of 
polysulphides  of  sodium  may  entirely  prevent  the  separation 

*For  the  preparation  of  this  salt,  see  section  on  Reagents. 


108         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

of  antimony,  or  cause  it  to  be  incomplete.  Inasmuch  as  the 
determination  of  antimony  is  ordinarily  preceded  by  its  sepa- 
ration from  metals  of  the  hydrogen  sulphide  group,  and  this 
separation  is  usually  accomplished  by  digestion  or  fusion  with 
polysulphides  of  the  alkalies,  the  quantitative  determination 
of  antimony  by  electrolysis  could  only  be  accomplished  if  the 
antimony  sulphide  precipitated  by  an  acid  from  the  sulpho- 
salt  were  first  freed  from  the  sulphur  precipitated  with  it.  It 
is  hardly  necessary  to  say  that  in  this  case  electrolysis  would 
have  no  advantage  over  other  methods  of  determination. 

In  a  paper  on  the  use  of  hydrogen  peroxide  in  analytical 
chemistry  *  the  author  has  referred  to  its  energetic  oxidizing 
action,  and  among  other  points  mentioned  that  it  \vill  con- 
vert monosulphides  of  the  alkalies  into  sulphates  without 
separation  of  sulphur.  Hydrogen  peroxide  reacts  with  the 
polysulphides  of  the  alkalies  in  the  same  way  as  with  the 
monosulphides. 

If  a  solution  of  sodium  pentasulphide  (obtained  by  fusing 
anhydrous  sodium  thiosulphate)  is  treated  with  ammoniacal 
hydrogen  peroxide,  and  heated,  the  dark  red  solution  becomes 
perfectly  colorless  without  separation  of  sulphur.  The 
method  of  determining  antimony  in  solutions  of  the  polysul- 
phides _,f  the  alkalies  thus  becomes  very  simple.  The  solution 
containing  polysulphides  is  treated  with  an  excess  of  hydrogen 
peroxide,  and  heated  till  it  becomes  colorless.  If  a  great 
excess  of  hydrogen  peroxide  is  used,  it  may  happen  that  the 
alkali  sulphide  is  entirely  decomposed,  and  antimony  sulphide 
precipitated.  If  the  solution  is  entirely  colorless,  or  if  a  pre- 
cipitate of  antimony  sulphide  has  already  appeared,  the  solu- 
tion is  cooled,  about  10  cc.  of  a  saturated  solution  of  sodium 
monosulphide  is  added,  and  the  cold  solution  of  150-175  cc. 

*  Ber.  d.  Ch.  Ges.,  16,  1062. 


DETERMINATION   OF  THE   METALS. 

volume  submitted  to  the  action  of  a  current  yielding  1.5-2  cc. 
oxyhydrogen  gas  per  minute.  If  the  electrolysis  is  begun  iri 
the  evening,  the  antimony  is  found  in  the  morning,  after  ten 
to  twelve  hours,  completely  precipitated.  If  the  weight  of 
the  antimony  does  not  exceed  0.16  gm.,  the  metal  adheres 
closely  to  the  dish  as  a  brilliant  grayish-white  coating. 

The  dish  with  the  deposit  of  antimony  is  washed  in  the 
usual  way  with  water  and  perfectly  pure  absolute  alcohol, 
dried  a  short  time  in  the  air-bath  at  80°-90°  C.,  and  weighed.* 

To  obtain  a  constant  current  of  1.5-2  cc.  oxyhydrogen  gas 
per  minute,  a  battery  of  four  to  six  Meidinger  cells  may  be  used. 

H.  Nissenson  f  adds  to  the  antimony  solution,  which  must 
contain  not  more  than  0.18  gm.,  50  cc.  sodium  sulphide  solu- 
tion^ and  electrolyzes  the  warm  solution  in  a  platinum  dish 
with  a  current  of  0.5-1  ampere.  Within  an  hour  about  0.15 
gm.  antimony  separates. 

F.  Riidorff  treats  the  antimony  solution  with  about  30  cc. 
of  a  10$  sodium  monosulphide  solution,  and  electrolyzes  with 
two  or  three  Meidinger  cells.  The  end  of  the  reaction  is  deter- 
mined by  testing  with  acetic  acid,  which  precipitaties  orange- 
red  antimony  sulphide,  if  the  reaction  is  incomplete. 

For  the  determination  of  antimony  as  amalgam,  G.  Yort- 
mann  gives  the  following  directions:  The  solution,  which 
must  contain  antimony  as  pentoxide,  and  to  that  end  must  be 
oxidized  with  bromine  water,  is  treated  with  a  weighed  quan- 
tity of  mercuric  chloride  (at  least  two  parts  of  mercury  to  one 
of  antimony),  and  the  solution  of  sodium  sulphide  and  sodium 
hydroxide  recommended  by  the  author  for  the  separation  of 
antimony  from  arsenic  (see  p.  139)  added  until  the  solution  is 

*  Classen  and  Ludwig,  Ber.  d.  Ch.  Ges.,  18,  1104. 

f  Personal  communication. 

\  The  commercial  preparation  is  treated  with  water  and  allowed  to 
stand  with  exclusion  of  the  air,  until  the  solution  becomes  colorless  above 
an  undissolved  residue. 


110         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

perfectly  clear.  It  is  diluted  to  about  175  cc.  and  electrolyzed 
according  to  directions  repeatedly  given  for  the  precipitation 
of  amalgams. 

[Chittenden  and  Blake*  have  applied  the  electrolytic  method 
to  the  determination  of  very  small  quantities  of  antimony  in  a 
large  amount  of  organic  matter.  In  test  experiments,  100  gm. 
of  beef  or  liver  were  finely  divided,  a  few  cc.  of  a  standard 
antimony  solution  added,  the  mixture  thoroughly  oxidized  with 
hydrochloric  acid  and  potassium  chlorate,  all  free  chlorine 
removed  by  heat,  and  the  antimony  precipitated  by  hydrogen 
sulphide.  The  precipitate  containing,  together  with  antimony 
sulphide,  some  sulphur  and  organic  matter,  was  dissolved  in 
cold  sodium  monosulphide,  and  directly  submitted  to  the 
action  of  a  current  from  four  gravity  cells  of  moderate  size. 
The  electrolytic  action  was  continued  till  all  the  organic  matter 
and  sulphur  was  oxidized  (eighteen  to  fei'ty-eight  hours),  and 
the  deposited  antimony  washed  without  breaking  the  current. 
Kesults  satisfactory,  and  much  better  than  those  obtained  by 
any  other  process. 

Chittenden  and  Blake  also  found  that  antimony  in  small 
quantities  was  deposited  quantitatively  from  urine  by  acidify- 
ing with  sulphuric  acid  (1  cc.  dilute  H2SO4  to  25  cc.  urine),  and 
submitting  directly  to  electrolysis.  The  battery  used  was  the 
same  as  before. — Trans.'} 

TIN. 

Tin  separates  completely  from  ammonium  double  oxalate, 
or  from  ammonium  sulphide  solution.  Sodium  and  potassium 
sulphides  cannot  be  used,  as  tin  separates  only  partially  from 
a  dilute  solution  of  the  corresponding  snlpho-salt,  and  not  at 
all  from  a  concentrated  solution. 

If  tin  is  precipitated  from  the  ammonium  double  oxalate, 
*  Trans.  Conn.  Acad.  Arts  and  Sci.,  7,  276. 


DETERMINATION   OF   THE   METALS.  Ill 

separation  of  stannic  acid  readily  occurs,  especially  when  much 
tin  is  present,  which  must  be  redissolved  by  addition  of  oxalic 
acid.  The  reduction  of  tin  proceeds,  however,  without  draw- 
backs, if  acid  ammonium  oxalate  is  used  instead  of  the  neutral 
oxalate.  The  results  obtained  by  this  process  are  so  accurate, 
that  the  author  has  found  it  adapted  to  the  determination  of 
the  atomic  weight  of  tin.* 

The  solution  of  tin  is  treated  with  a  cold  saturated  solution 
of  acid  ammonium  oxalate  in  the  proportion  of  20  cc.  to  0.1 
gm.  tin.  The  solution  is  diluted  to  about  150  cc.,  heated,  and 
•electrolyzed  with  a  current  of  2.5-3  cc.  OH  gas,  which  is 
increased  toward  the  end  to  5  cc.  The  tin  is  completely 
t precipitated  as  a  closely  adherent  shining  silver  white  metal, 
even  when  as  much  as  6  gm.  is  present.  The  current  is  inter- 
rupted, and  the  metal  washed  as  usual  with  water  and  alcohol, 
and  dried  at  80°-90°. 

In  the  solution  of  the  ammonium  sulpho-salt  tin  behaves 
like  antimony.  The  tin  solution  (if  necessary  after  neutraliza- 
tion with  ammonia)  is  treated  with  ammonium  sulphide  free 
from  ammonia  (no  more  is  added  than  is  needed  to  form  the 
sulpho-salt),  diluted  to  150-175  cc.,  and  electrolyzed  with  a 
current  of  the  strength  already  given.  The  tin  is  all  precipi- 
tated in  five  to  six  hours.  Sometimes  a  deposit  of  sulphur 
adheres  so  strongly  to  the  tin  at  the  edge  of  the  dish  that  it 
cannot  be  washed  off  with  water ;  it  may,  however,  be  easily 
removed,  after  washing  with  alcohol,  by  gentle  rubbing  with 
a  linen  cloth.  This  method  of  determining  tin  is,  however, 
less  convenient  and  accurate  than  the  former. 

In  gravimetric  analysis  tin  is  often  separated  from  other 
metals  by  sodium  sulphide  instead  of  ammonium  sulphide. 
In  order  to  determine  the  tin  electrolytically  in  such  cases, 

*  Bongartz  and  Classen,  B.  d.  Ch.  Ges.,  21,  2900. 


112        QUANTITATIVE   ANALYSIS   EY    ELECTROLYSIS. 

the  sodium  sulphide  must  be  converted  into  ammonium 
sulphide.*  To  accomplish  this,  the  solution  is  treated  with 
about  «25  gm.  pure  ammonium  sulphate  free  from  iron,  and 
heated  very  carefully,  with  the  dish  covered,  till  the  hydrogen 
sulphide  has  all  escaped ;  the  solution  is  then  kept  in  gentle 
ebullition  for  about  fifteen  minutes.  Complete  conversion 
into  ammonium  sulphide  is  shown  by  the  greenish  yellow 
color  of  the  solution.  If  the  heating  is  continued  too  long, 
tin  sulphide  may  separate;  it  can  be  dissolved  in  ammonium 
sulphide.  After  it  is  completely  cooled,  any  sodium  sulphate 
that  may  have  separated  is  dissolved  by  addition  of  water, 
and  the  solution  electrolyzed  with  a  current  of  9-10  cc.  oxy- 
hydrogen  gas  per  minute. 

The  determination  of  the  tin  is  much  more  simply  and 
easily  accomplished  by  converting  the  solution  of  tin  sulphide 
in  sodium  sulphide  into  the  acid  oxalate.  This  conversion 
may  be  accomplished  in  two  ways:  either  the  sulpho-salt  is 
decomposed  with  dilute  sulphuric  acid  to  remove  the  greater 
part  of  the  sulphur  as  hydrogen  sulphide,  and  the  separated 
tin  sulphide  oxidized  with  hydrogen  peroxide  f  until  the 
stannic  acid  which  is  produced  appears  clear  white,  or  the 
heated  alkaline  solution  is  treated  directly  with  hydrogen 
peroxide  (of  which  a  great  quantity  is  needed),  then  acidified 
with  sulphuric  acid  to  precipitate  stannic  acid,  neutralized 
with  ammonia,  and  treated  with  more  hydrogen  peroxide. 
In  either  case  the  solution  is  heated  to  decompose  the  excess 
of  hydrogen  peroxide,  and  the  stannic  acid  allowed  to  settle 
and  then  filtered  off.  The  precipitate  is  washed  from  the 
filter  with  the  oxalate  solution  into  a  beaker,  the  filter  washed 

*  Sodium  sulphide  cannot  be  replaced  by  potassium  sulphide  in  the 
separation  from  other  metals,  because  the  latter  produces  difficultly  soluble 
potassium  sulphate  when  ammonium  sulphide  is  formed. 

f  Classen  and  Bauer,  Ber.  d.  Ch.  Ges.,  16,  1062. 


DETERMINATION    OF   THE   METALS.  113 

with  hot  oxalic  acid  solution,  and  the  stannic  acid  in  the  beaker 
dissolved  by  heating.  Sometimes  there  is  a  residue  of  sulphur, 
which  is  removed  by  filtration.  The  filtrate  is  collected  in  the 
weighed  platinum  dish  to  be  used  for  the  electrolysis,  and  the 
sulphur  is  washed  with  a  cold  saturated  solution  of  acid  ammo- 
nium oxalate.  The  solution  for  electrolysis  must  contain  at 
least  50  cc.  of  this  solution. 

ARSENIC. 

Arsenic  cannot  be  fully  precipitated  from  solution  in 
water,  in  hydrochloric  acid,  or  in  excess  of  ammonium  oxalate 
or  alkaline  sulphides.  A  part  of  the  metal  is  reduced  from 
aqueous  or  oxalate  solution,  while  by  long-continued  action  of 
the  current  all  the  arsenic  is  driven  off  from  hydrochloric  acid 
solution  as  hydrogen  arsenide. 

The  action  of  arsenic  (present  as  arsenic  acid)  in  a  concen- 
trated solution  of  sodium  sulphide  allows  of  its  separation  from 
antimony,  as  described  later. 

POTASSIUM.     AMMONIA.     (NITROGEN.) 

Potassium  and  ammonia  may  be  determined,  as  is  well 
known,  by  converting  them  into  potassium  or  ammonium 
platinchloride,  and  weighing  the  precipitate,  dried  at  110°,  on 
a  tared  filter.  This  method,  which  is  almost  universally  em- 
ployed in  the  separation  of  potassium  from  sodium,  has  many 
disadvantages.  It  is  preferable,  after  precipitating  and  wash- 
ing the  platinum  salt  as  usual,  to  dissolve  it  in  water,  and  de- 
termine the  platinum  as  directed  on  p.  104. 

DETERMINATION  OF  NITRIC  ACID  IN  NITRATES. 

As  is  well  known,  nitric  acid  is  often  converted  into  am- 
monia, and  the  latter  determined.  The  action  of  the  galvanic 


114         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

.• 

current  converts  nitric  acid  into  ammonia,  as  explained  in  the 
Introduction  (p.  3).  If  the  solution  of  an  alkali  nitrate,  acidi- 
fied with  dilute  sulphuric  acid,  is  exposed  to  the  action  of  the 
galvanic  current,  no  ammonia  is  formed. 

Luckow  discovered  that  reduction  of  the  nitric  acid  always 
takes  place  when  a  salt  from  which  the  metal  is  precipitated 
by  the  current  is  also  present  in  the  solution.  Copper  salts 
are  best  adapted  for  this  purpose.  G.  Vortmann  has  deter- 
mined in  the  Aachen  laboratory  the  conditions  for  the  quanti- 
tative determination  of  nitric  acid  in  nitrates.  The  solution 
of  the  nitrate  is  treated  with  a  sufficient  quantity  of  copper 
sulphate  (in  the  analysis  of  potassium  nitrate,  e.g.,  half  as 
much  crystallized  copper  sulphate  as  of  the  nitrate),  acidified 
with  dilute  sulphuric  acid,  and  electrolyzed  cold  with  a  cur- 
rent of  1-2  cc.  electrolytic  gas.  When  the  reaction  is  com- 
plete the  solution  is  poured  off,  and  sodium  hydroxide  solution 
is  added,  and  the  ammonia  distilled  off  and  determined  volu- 
metrically  in  the  usual  way.  For  this  purpose  one-fifth  nor- 
mal solutions  of  ammonia  and  sulphuric  acid  are  used.  To 
standardize  the  sulphuric  acid,  a  weighed  quantity  (0.5  gm.) 
of  crystallized  copper  sulphate  is  decomposed  electrolytically, 
and  the  resulting  free  acid  titrated  with  ammonia.  G.  Yort- 
mann  decomposed,  e.g.,  0.4876  gm.  CuSO4,  5H2O,  and  used, 
for  the  neutralization  of  the  acid  set  free,  19.6  cc.  of  ammonia 
of  equal  strength  with  the  one-fifth  normal  sulphuric  acid, 
1  cc.  of  the  latter  corresponds  therefore  to  0.0028017  gm.  of 
nitrogen  in  the  form  of  ammonia. 


SEPARATION   OF  THE  METALS.  115 


SEPARATION  OF  THE  META 


IRON  AND  COBALT. 

THE  two  metals  may  be  determined  by  electrolyzing  the 
solution  of  the  double  oxalates,  as  directed  under  Iron  (p.  78), 
weighing  the  iron  and  cobalt  together,  and  determining  the 
former  volumetrically. 

After  weighing  the  iron  and  cobalt,  the  deposit  is  dissolved 
in  dilute  sulphuric  acid  (dilute  sulphuric  acid  is  poured  over, 
and  concentrated  acid  gradually  added,  so  that  the  solution 
becomes  heated),  and  the  iron  is  titrated  in  the  platinum  dish 
with  potassium  permanganate.  To  overcome  the  red  color 
of  cobalt,  sulphate,  a  sufficient  amount  of  nickel  sulphate  is 
added  before  the  titration.  The  end  of  the  reaction  is  easily 
seen. 

The  residue  of  cobalt  and  iron  may  also  be  dissolved  in 
hydrochloric  acid,  the  iron  oxidized  with  hydrogen  peroxide, 
and  titrated,  after  removing  the  excess  of  hydrogen  peroxide 
by  boiling,  with  stannous  chloride. 

IRON  AND  NICKEL. 

The  method  of  determination  is  exactly  like  the  preceding. 
Iron  and  nickel  separate  in  the  form  of  a  beautiful  white  alloy 
scarcely  distinguishable  from  the  platinum.  This  alloy  resists 
strongly  the  action  of  acids,  and  is  only  very  slowly  attacked 
by  dilute  sulphuric  or  hydrochloric  acid. 

To  determine  the  iron,  the  precipitate  in  the  dish  must  be 
heated  with  concentrated  hydrochloric  acid ;  and,  if  the  iron 
is  to  be  titrated  with  permanganate,  the  solution  must  be  re- 


116         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

duced  by  nascent  hydrogen.  It  is  more  simple  to  oxidize 
with  hydrogen  peroxide,  and,  after  removing  the  excess,  ti- 
trate the  ferric  chloride  with  stannous  chloride. 

IRON  FROM  COBALT  AND  NICKEL. 

G.  A.  LeRoy  proposes  the  following  method :  The  sul- 
phuric acid  solution  of  the  metals,  containing  the  iron  in  the 
ferric  state,  is  treated  with  as  little  citric  acid  as  possible  to 
prevent  precipitation  by  ammonia,  then  with  a  great  excess  of 
concentrated  ammonium  sulphate  solution  containing  free 
ammonia.  The  solution  is  electrolyzed  with  a  current  disen- 
gaging 5  cc.  electrolytic  gas  per  minute,  and  the  metals  are 
deposited  on  the  negative  electrode.  After  weighing,  a 
strongly  ammoniacal  ammonium  sulphate  solution  is  poured 
over  the  metals,  the  dish  is  connected  with  the  positive  pole 
of  the  source  of  current,  and  a  current  of  about  1.5  cc.  oxyhy- 
drogen  gas  is  passed.  Nickel  and  cobalt  are  now  deposited  on 
the  platinum  foil  forming  the  negative  electrode,  while  the 
iron  is  converted  into  hydroxide,  and  in  part  adheres  to  the 
dish,  in  part  is  suspended  in  the  liquid. 

IRON  AND  ZINC. 

If  the  double  oxalates  of  iron  and  zinc  are  submitted  to 
electrolysis,  an  alloy  of  the  two  does  not  separate,  but  zinc, 
with  a  little  iron,  is  first  precipitated  on  the  negative  elec- 
trode. The  electrolysis  proceeds  very  satisfactorily,  and  the 
united  weight  of  the  two  metals  may  readily  be  determined,  if 
there  is  less  than  one-third  as  much  zinc  as  iron  in  the  solu- 
tion. If  the  proportion  of  zinc  is  greater,  the  zinc  dissolves 
with  the  evolution  of  gas  as  the  action  proceeds,  and  a  pre- 
cipitate of  iron  oxide  is  formed. 

When  zinc  is  present  in  too  large  proportion,  it  is  only 
necessary  to  add  a  weighed  quantity  of  a  pure  iron  salt  (e.g., 
ammonium  ferrous  sulphate,  which  contains  one-seventh  its 


SEPARATION    OF   THE   METALS.  117 

weight  of  iron)  to  the  solution  in  order  to  determine  both 
metals. 

IRON,  COBALT,  NICKEL,  AND   ZINC,  FROM  ALUMINIUM. 

When  a  solution  containing  the  above-named  metals  and 
a  great  excess  of  ammonium  oxalate  is  electrolyzed  in  the 
cold,  iron,  cobalt,  nickel,  or  zinc,  or  all  four  metals,  if  present, 
are  deposited  on  the  negative  electrode,  while  the  aluminium 
remains  in  solution  as  long  as  arnmoniu'm  oxalate  is  present 
in  the  solution  in  greater  proportion  than  the  ammonium 
carbonate  formed  from  it.  If  a  precipitate  of  aluminium 
hydroxide  finally  appears,  it  is  only  when  the  solution  is 
almost  free  from  the  other  metals.  A  small  portion  with- 
drawn by  a  capillary  tube  is  tested,  from  time  to  time,  with 
ammonium  sulphide  or  another  reagent  already  mentioned, 
and  the  current  is  stopped  as  soon  as  no  reaction  is  obtained. 

The  process  is  as  follows :  The  aqueous  or  weakly  acid 
solution  (in  the  latter  case  neutralized  with  ammonia)  of  the 
sulphates  (the  chlorides  are  not  as  well  adapted  to  the  process) 
is  treated  with  ammonium  oxalate  in  excess  (the  entire 
volume  of  the  solution  should  be  150-175  cc.),  and  enough 
solid  ammonium  oxalate  added  (with  gentle  warming  if  neces- 
sary) to  give  the  proportion  of  2-3  grn.  ammonium  oxalate  to 
0.1  gm.  of  the  metals.  If  the  temperature  of  the  solution  is 
not  over  40°,  it  may  be  submitted  to  electrolysis  at  once,  since 
it  gradually  cools  under  the  action  of  a  current  of  the  given 
strength. 

It  is  not  best  to  continue  the  action  of  the  current  longer 
than  is  necessary  to  reduce  the  iron,  cobalt,  nickel,  and  zinc ; 
for,  otherwise,  a  large  part  of  the  aluminium  is  precipitated 
as  hydroxide,  and  clings  so  closely  to  the  negative  electrode 
that  it  cannot  be  removed. 

In   such  a  case   it  is   necessary  to  bring  the   aluminium 


118        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

hydroxide  into  solution  by  acidifying  with  oxalic  acid,  and,  in 
case  too  much  acid  has  been  added,  to  pass  the  current  till 
the  last  traces  of  the  redissolved  metals  have  been  again  pre- 
cipitated. 

The  oxalic  acid  is  poured  gradually  down  the  glass  which 
covers  the  platinum  dish,  without  interrupting  the  current, 
till  there  is  no  more  ebullition,  and  the  aluminium  precipitate 
is  dissolved. 

If  the  quantity  of  the  aluminium  is  not  greater  than  that 
of  the  other  metals,  the  method  gives  good  results  without 
further  treatment.  In  other  cases,  the  precipitate  of  alumin- 
ium hydroxide  is  dissolved,  without  interrupting  the  current, 
by  careful  addition  of  oxalic  acid,  and  the  electrolysis  repeated 
till  the  metals  to  be  separated  are  fully  precipitated.*  To 
determine  the  aluminium  in  the  solution  poured  off  from  the 
iron,  cobalt,  etc.,  it  is  heated  in  a  porcelain  dish  till  the  am- 
monia is  driven  off,  filtered,  and  the  aluminium  hydroxide 
converted,  by  ignition,  into  A12O3. 

It  may  be  remarked  that  experiments  looking  toward  the 
reduction  of  aluminium  oxide  to  metal,  and  quantitative  de- 
termination of  the  latter,  are  thus  far  fruitless. 

IRON  FROM  MANGANESE. 

. 

A  solution  of  ammonium  oxalate  is  decomposed  by  elec- 
trolysis, as  stated  in  the  introduction,  mainly  into  hydrogen 
r^    and    hydrogen  ammonium    carbonate.     The   latter  is   partly 
decomposed  into  ammonia,  which  mostly  remains  in  solution, 

*  According  to  later  researches  (Classen,  Ber.  d.  Ch.  Ges.,  18,  1795), 
•fer.^fae  separation  may  be  made  complete  in  one  operation,  if  care  is  taken  that 
the  ammonium  oxalate  is  not  unnecessarily  decomposed.  This  is  easily 
accomplished  by  using  a  great  excess  of  ammonium  oxalate,  electrolyziug 
in  the  cold,  and  not  employing  a  current  so  strong  as  to  decompose  the 
double  oxalate  with  great  rapidity.  No  aluminium  hydroxide  separates 
•with  a  current  of  10-12  cc.  oxyhydrogen  gas. 


SEPARATION    OF    THE    METALS.  119 

and  carbon  dioxide.  In  the  electrolysis  of  a  hot  solution  of 
ammonium  oxalate,  the  ammonium  carbonate  produced  by 
the  current  is  partly  neutralized  as  a  result  of  dissociation  of 
ammonium  oxalate;  carbon  dioxide  is  rapidly  liberated  at 
the  positive  electrode. 

If  a  solution  of  the  double  oxalates  of  iron  and  manganese 
is  subjected  to  electrolysis  without  the  previous  addition  of 
a  great  excess  of  ammonium  oxalate,  the  characteristic  color 
of  permanganic  acid  appears  immediately  at  the  positive 
electrode,  and  manganese  dioxide  gradually  separates  at  the 
positive  electrode,  and  iron  at  the  negative.  If  the  elec- 
trolysis is  conducted  under  these  conditions,  it  is  impossible 
to  obtain  a  quantitative  separation  of  the  two  metals,  because 
the  manganese  dioxide  carries  down  with  it  considerable 
quantities  of  ferric  hydroxide.  The  complete  separation  of 
the  metals  is  possible  only  when  the  separation  of  the  man- 
ganese dioxide  is  delayed  till  most  of  the  iron  is  precipitated. 
If  a  solution  of  the  double  oxalates  of  iron  and  manganese, 
which  contains  a  great  excess  of  ammonium  oxalate,  is  elec- 
trolyzed  in  the  cold,  the  greater  part  of  the  manganese  dioxide 
is  precipitated  only  after  most  of  the  ammonium  oxalate  is 
decomposed.  In  this  case,  however,  the  separation  of  the 
manganese  dioxide  is  incomplete,  because,  by  the  action  of 
the  great  quantity  of  ammonium  carbonate  or  ammonia  that 
is  produced  on  the  manganese  double  salt,  a  considerable 
portion  of  the  precipitate  (a  mixture  of  dioxide  and  a  lower 
oxide)  goes  into  solution. 

The  rapid  dissociation  of  ammonium  oxalate  when  heated, 
however,  gives  a  simple  means  of  delaying,  or  entirely  pre- 
venting, the  formation  of  a  manganese  precipitate  during 
electrolysis.  A  solution  of  iron  and  manganese,  with  large 
excess  of  ammonium  oxalate  (the  process  of  conversion  into 
the  double  salt  is  conducted  as  on  p.  79.  except  that  8-10  gin. 


120  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

ammonium  oxalate  are  added  to  the  solution),  is  heated  (see 
p.  76),  and  exposed  to  the  action  of  a  current  of  10-12  cc. 
oxyhydrogen  gas  per  minute.  The  separation  from  mangan- 
ese is  complete ;  only  a  slight  deposit  of  dioxide  appears  on 
the  positive  electrode,  even  when  the  quantity  of  manganese 
is  large,  and  the  solution  is  scarcely  turbid. 

When  the  reduction  is  complete,  the  solution  is  poured 
off,  the  dish  washed  repeatedly  with  water,  and  this,  together 
with  traces  of  the  dioxide  precipitate,  removed  by  alcohol ; 
it  is  sometimes  necessary  gently  to  rub  the  dish  with  the 
finger. 

The  process  is  so  simple  that  unskilled  workers  easily 
obtain  close  results.  To  test  this,  a  student  in  the  author's 
laboratory,  who  had  never  done  quantitative  work,  was  in- 
structed in  the  details  of  the  process ;  he  obtained  results 
differing  only  by  0.05  per  cent  from  the  original  weight. 

As  has  already  been  stated  in  treating  of  the  quantitative 
determination  of  manganese  as  dioxide,  it  cannot  be  com- 
pletely precipitated  from  a  solution  in  ammonium  oxalate. 

To  obtain  a  complete  separation,  the  solution,  containing 
suspended  manganese  dioxide,  is  heated  with  solution  of  pure 
potassium  or  sodium  hydroxide,  in  a  porcelain  dish,  till  the 
ammonium  carbonate  produced  by  electrolysis  is  decomposed, 
and  the  solution  no  longer  smells  of  ammonia ;  and  then 
sodium  carbonate  and  a  little  sodium  hypochlorite,  or  better, 
hydrogen  peroxide,  are  added.  The  manganese  dioxide 
quickly  falls  to  the  bottom,  and  can  be  filtered  off.  The 
precipitate  is  best  washed  with  hot  water,  to  which  a  little 
ammonium  nitrate  has  been  added,  and  is  either  converted 
into  mangano-manganic  oxide  (Mn3O4)  by  ignition,  or,  bet- 
ter, into  manganese  sulphate  (MnSO4). 

The  conversion  into  manganese  sulphate  is  accomplished 
by  moistening  the  precipitate  in  the  crucible  with  a  little 


SEPAEATION   OF  THE  METALS. 

pure  concentrated  sulphuric  acid,  and  igniting  very  gently, 
so  that  the  bottom  of  the  crucible  is  barely  red. 

If  it  is  desired  to  determine  the  manganese  as  manganese 
sulphide,  the  solution  is  boiled  till  the  ammonium  carbonate 
is  decomposed,  the  remaining  ammonia  is  neutralized  with 
nitric  acid,  and  ammonium  sulphide  added  till  the  precipi- 
tation is  complete.  The  manganese  sulphide  is  either  deter- 
mined as  such,  by  ignition  in  a  stream  of  hydrogen,  or,  more 
simply,  converted  into  manganese  sulphate  by  heating  with 
.a  few  drops  of  sulphuric  acid. 

A.  Brand  has  developed  a  suggestion  of  the  author  in 
1881  *  as  to  the  separation  of  manganese  from  iron  and 
cobalt ;  he  conducts  the  process  as  follows :  The  solution  is 
treated  with  a  great  excess  of  sodium  pyrophosphate,  so  that 
a  clear  solution  results,  and  4  to  8  gms.  ammonium  oxalate 
dissolved  in  it.  Strongly  acid  solutions  must  be  neutralized 
before  adding  sodium  pyrophosphate.  The  electrolysis  is  con- 
ducted with  a  current  of  5  cc.  electrolytic  gas  per  minute, 
which  is  increased  toward  the  end  to  15-20  cc. 

To  determine  the  manganese  the  solution  is  made  weakly 
acid  with  sulphuric  acid,  and  the  red  manganic  double  salt 
reduced  by  heating  with  oxalic  acid.  Then  15$  of  concen- 
trated ammonia  is  added,  and  the  manganese  determined  ac- 
cording to  Brand's  method  (p.  87). 

NICKEL   FROM   MANGANESE. 

The  separation  is  accomplished  exactly  like  that  of  iron 
from  manganese. 

*  "  The  separation  of  the  two  metals  is  possible  only  when  the  formation 
of  manganese  peroxide  is  delayed  until  the  greater  part  of  the  iron  is  pre- 
cipitated. This  may  be 'accomplished  by  the  addition  of  sodium  phos- 
phate, or,  more  conveniently,  by  a  great  excess  of  ammonium  oxalate." 
Ber.  d.  Ch.  Ges.  14,  1630. 


122        QUANTITATIVE  ANALYSIS  BY   ELECTROLYSIS. 


COBALT  AND   ZINC   FROM   MANGANESE. 

Cobalt  and  zinc  are  not  well  adapted  for  precipitation 
from  hot  solution,  because,  when  a  current  of  about  10  cc.  is. 
used  (a  strength  sufficient  for  reduction  in  the  cold),  the 
metals  separate  in  a  spongy  state,  and  do  not  adhere  to 
the  platinum.  If  the  two  metals  are  to  be  separated  from 
manganese,  therefore,  the  electrolysis  of  the  double  salts 
(prepared  as  heretofore  described)  is  conducted  at  the  ordi- 
nary temperature.  If  the  quantity  of  manganese  is  small,, 
the  separation  is  rapid  and  complete.  If  the  quantity  of 
manganese  is  large,  the  reduction  is  slower ;  it  is  then  neces- 
sary, to  obtain  complete  separation,  to  dissolve  the  precipitated 
dioxide,  without  interrupting  the  current,  in  oxalic  acid,  and 
continue  the  action  of  the  current.  The  process  is  the  same 
as  in  the  separation  of  aluminium  from  iron,  etc. 

NICKEL,    COPPER,    CADMIUM,   ZINC,   AND   MERCURY 
FROM   MANGANESE. 

According  to  A.  Brand  nickel  may  be  separated  from 
manganese  by  converting  the  metals  into  double  salts  by  the 
addition  of  an  excess  of  sodium  phosphate,  adding  15$  of  con- 
concentrated  ammonia,  and  electrolyzing.  The  separation  of 
nickel  begins  with  a  weak  current;  toward  the  end  of  the 
reduction  the  current  is  increased  to  some  10  cc.  oxyhydrogen 
gas  per  minute.  The  platinum  dish  is  made  the  positive 
electrode,  and  the  nickel  deposited  on  a  stout  platinum  wire. 

If  copper  and  manganese  are  to  be  separated,  the  electroly- 
sis must  be  begun  with  a  very  weak  current,  that  is  gradu- 
ally increased  in  strength.  The  method  is  desirable  only  for 
small  quantities  of  the  metals.  The  process  for  the  separation 
of  cadmium,  zinc,  and  mercury  from  manganese  resembles- 
that  for  the  separation  of  nickel  and  manganese. 


SEPARATION    OF   THE   METALS.  128 

MANGANESE    FROM   COPPER,   CADMIUM,   MERCURY. 

The  method  is  based  on  the  precipitation  of  copper,  cad- 
mium, and  mercury  in  the  presence  of  free  pyrophosphoric 
acids,  or,  better,  from  solution  of  the  pyrophosphates  acidified 
with  sulphuric  or  nitric  acid.  By  the  addition  of  sodium 
pyrophosphate  in  excess  the  metals  are  converted  into  double 
salts,  the  solution  is  acidified  with  sulphuric  or  nitric  acid,  and 
submitted  to  electrolysis.  Copper  is  precipitated  with  a  cur- 
rent of  3-4  cc.  electrolytic  gas  per  minute,  mercury  with  one 
of  0.2-0.5  cc.,  cadmium  with  the  same,  which,  however,  toward 
the  end  of  the  reaction  must  be  increased  to  5-10  cc. 

In  the  solution,  freed  from  the  foregoing  metals,  the  man- 
ganese is  reduced,  as  already  directed,  with  oxalic  acid,  and 
separated  as  peroxide  from  ammoniacal  solution  (see  p.  87). 
(A.  Brand.) 

IRON,    COBALT,    NICKEL,   AND    ZINC    FROM    MANGANESE 
AND   ALUMINIUM. 

The  separation  from  the  double  oxalate  solution  is  con- 
ducted like  the  separation  of  cobalt  and  zinc  from  manganese, 
already  described.  When  the  reduction  is  complete,  the  solu- 
tion containing  the  manganese  precipitate  is  boiled  with  an 
excess  of  sodium  hydroxide  solution,  a  few  cc.  of  sodium  hy- 
pochlorite  or  hydrogen  peroxide  are  added,  and  the  manga- 
nese dioxide  immediately  filtered  off. 

The  filtrate  is  acidified  with  hydrochloric  acid,  the  alu- 
minium precipitated  as  hydroxide  by  ammonia,  and  deter- 
mined, as  usual,  as  aluminium  oxide. 

IRON,  COBALT,  NICKEL,  AND  ZINC  FROM  CHROMIUM. 

If  a  solution  which  contains  an  excess  of  ammonium 
oxalate,  and  chromium  as  sesquioxide,  that  is,  as  chromium 


124         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

ammonium  oxalate,  is  submitted  to  electrolysis,  the  chromium 
is  all  converted  into  a  chromate.  If  iron,  cobalt,  etc.,  are  also 
present,  they  are  first  precipitated  in  the  metallic  state  on 
the  negative  electrode ;  the  metals  have  a  peculiarly  distinct 
lustre. 

When  the  precipitation  is  complete,  the  solution  is  boiled 
to  decompose  ammonium  carbonate,  the  chromic  acid  reduced 
by  boiling  with  hydrochloric  acid  and  alcohol,  and  the  chro- 
mium precipitated  as  hydroxide  with  ammonia. 

The  hydroxide  is  converted  into  Cr2O3  in  the  usual  manner, 
and  weighed. 

IRON,    COBALT,    NICKEL,     AND     ZINC    FROM     CHROMIUM 
AND  ALUMINIUM. 

The  separation  is  performed  as  above.  To  separate  the 
aluminium  from  chromium,  the  solution  poured  off  from  the 
precipitated  metals  is  boiled  till  it  has  only  a  weak  odor  of 
ammonia,  the  aluminium  hydroxide  filtered  off,  and  the  chro- 
mium precipitated  as  above. 

IRON,    COBALT,   NICKEL,   AND    ZINC   FROM   MANGANESE, 
CHROMIUM,  AND  ALUMINIUM. 

The  process  is  conducted  substantially  as  above.  When 
the  yellow  color  of  chromic  acid  appears,  the  solution  is 
poured  off,  boiled  to  decompose  the  ammonium  carbonate, 
and  precipitated  hot  with  sodium  hydroxide,  with  the  addi- 
tion of  a  little  sodium  hypochlorite  or  hydrogen  peroxide. 
The  manganese  dioxide  precipitate  always  contains  some 
chromium.  It  is  filtered,  washed,  dissolved  in  hydrochloric 
acid,  and  the  precipitation  with  sodium  hydroxide  and  an 
oxidizing  agent  repeated.  Chromium  is  determined,  as  above, 
in  the  filtrate  from  the  manganese  dioxide. 


SEPARATION    OF   THE   METALS. 


IRON,   COBALT,   NICKEL,  AND  ZINC   FROM   URANIUM. 

The  separation  from  uranium  depends  on  the  same  prin- 
ciple as  that  from  aluminium.  It  is  necessary  to  have  a  great 
excess  of  ammonium  oxalate  in  order  to  keep  the  uranium  in 
solution  as  double  salt  till  the  other  metals  are  precipitated. 
A  current  of  10  or  12  cc.  oxyhydrogen  gas  per  minute  is 
employed;  with  a  stronger  current  it  may  happen  that  the 
uranium  is  precipitated  as  hydroxide,  as  a  result  of  the  de- 
composition of  hydrogen  ammonium  carbonate  by  the  heat 
produced.  The  uranium,  after  the  separation  of  the  other 
metals,  is  precipitated  by  the  further  decomposition  of  the 
oxalate,  and  the  ammonium  carbonate  is  finally  decomposed 
by  heat.  To  bring  the  finely  divided  precipitate  of  uranium 
hydroxide  into  suitable  condition  for  filtration,  nitric  acid  is 
added,  the  solution  is  heated  till  the  precipitate  is  wholly  dis- 
solved, and  ammonia  is  added  to  reprecipitate  the  hydroxide. 
The  precipitate  is  converted  into  uranium  oxide  by  ignition  in 
a  stream  of  hydrogen. 

IRON,    COBALT,   NICKEL,   AND    ZINC   FROM   CHROMIUM 
AND   URANIUM. 

The  separation  is  accomplished  by  the  precipitation  of 
iron,  cobalt,  nickel,  and  zinc  from  the  double  oxalate  solu- 
tion, and  the  oxidation  of  chromium  to  chromic  acid  by  the 
current.  Uranium  is  separated  as  hydroxide,  while  chromium 
remains  in  solution  as  ammonium  chromate.  To  accomplish 
the  quantitative  separation  of  chromium  from  uranium,  the 
electrolysis  must  be  continued  till  the  oxalic  acid  is  completely 
oxidized. 

The  solution  is  boiled  to  decompose  the  resulting  ammo- 
nium carbonate,  and  allowed  to  stand  six  hours.  The  chromi- 
um is  determined,  as  above,  in  the  filtrate  from  the  uranium. 


126         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

IRON,    NICKEL,    COBALT,    AND    ZINC    FROM    ALUMINIUM, 
MAGNESIUM,  AND  URANIUM. 

The  method  proposed  by  A.  Brand  depends  on  the  pre- 
cipitation of  the  first  four  metals,  while  the  rest  remain  in 
solution.  The  metals  are  converted  into  double  pyrophos- 
phates  by  treatment  with  sodium  pyrophosphate  in  excess,  and 
ammonium  carbonate  added  in  slight  excess.  The  electrolysis 
is  conducted  according  to  directions  already  given  for  the  in- 
dividual metals.  This  method  is  not  adapted  to  cases  in 
which  aluminium,  magnesium,  and  uranium  are  to  be  deter- 
mined, since  the  presence  of  pyrophosphoric  acid  makes  these 
•determinations  difficult. 

MANGANESE   FROM   BARIUM,  STRONTIUM,    CALCIUM, 
MAGNESIUM,  AND  THE  ALKALIES. 

The  manganese  can  be  easily  separated  from  the  other 
metals  by  precipitating  it,  as  dioxide,  by  electrolysis.  If  cal- 
cium is  precipitated  as  oxalate  from  a  solution  containing 
manganese,  it  carries  down  a  portion  of  the  latter,  as  is  well 
known,  as  manganese  oxalate. 

The  amount  of  manganese  in  the  ignited  and  weighed  pre- 
cipitate (CaO  -f-  Mn,O3)  may  be  easily  determined  by  dissolv- 
ing it  in  nitric  acid  and  electrolyzing  (see  Manganese). 

IRON  AND  BERYLLIUM. 

The  separation  of  these  two  metals  offers  no  difficulties 
whatever  if  the  soluble  double  salts  are  prepared  by  the  use 
of  ammonium  oxalate,  with  no  potassium  oxalate,  if  care  is 
taken  to  have  an  excess  of  ammonium  oxalate,  and  if  the  iron 
is  precipitated  by  a  current  of  10-12  cc.  oxy hydrogen  gas  per 
minute ;  a  stronger  current  is  not  advisable  lest  the  solution 
become  heated,  and  thus  the  ammonium  carbonate,  which 


SEPARATION   OF  THE   METALS.  127 

holds  the  beryllium  in  solution,  be  decomposed.  The 
beryllium  hydroxide  may,  in  any  case,  begin  to  be  precipi- 
tated before  the  iron  is  fully  deposited.  The  determination 
of  beryllium  in  the  solution  poured  off  from  the  iron  is  very 
simple ;  the  solution  is  boiled  to  decompose  the  hydrogen 
ammonium  carbonate,  and  the  heating  continued  till  the  solu- 
tion has  only  a  weak  odor  of  ammonia.  The  beryllium  hy- 
droxide is  filtered,  washed  with  hot  water,  and  converted  into 
BeO  by  ignition  in  a  platinum  crucible. 

IRON   FROM    BERYLLIUM   AND   ALUMINIUM. 

The  process  is  precisely  like  the  foregoing.  When  the 
iron  is  reduced,  the  solution  is  poured  into  a  second  platinum 
dish,  and  the  action  of  a  current  of  10-12  cc.  oxyhydrogen 
gas  per  minute  is  continued  till  all  the  oxalic  acid  is  decom- 
posed, and  the  aluminium  is  precipitated  as  hydroxide.  The 
beryllium  is  precipitated  from  the  filtrate  as  hydroxide  by 
boiling. 

It  is  advisable  to  redissolve  the  aluminium  hydroxide, 
convert  it  again  into  the  double  oxalate,  and  repeat  the 
electrolysis. 

IRON  AND  ZIRCON. 

The  separation  and  the  estimation  of  the  zircon  are  con- 
ducted exactly  as  in  the  case  of  beryllium. 

IRON   AND  VANADIUM. 

The  separation  is  as  perfect  as  that  of  iron  from  beryllium. 

IRON,  MANGANESE,  AND   PHOSPHORIC   ACID. 

The  iron  and  manganese  are  separated  as  directed  on 
p.  118,  the  iron  being  separated  as  metal,  and  the  manganese 
as  dioxide.  The  filtrate  from  the  latter  contains  all  the 


128        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

phosphoric  acid.  It  is  determined  by  acidifying  with  hydro- 
chloric acid,*  and  adding  one-third  the  volume  of  ammonia, 
and  then  magnesium  chloride  solution.  The  precipitate  of 
magnesium  ammonium  phosphate  is  converted  into  magne- 
sium pyrophosphate  as  usual. 

The  foregoing  process  must  also  be  followed  when  only 
phosphoric  acid  is  to  be  determined  in  the  presence  of  iron 
and  manganese.  If  the  solution,  when  free  from  iron  and 
a  part  of  the  manganese  (that  is,  without  treatment  with 
sodium  hypochlorite  or  hydrogen  peroxide),  were  to  be  used 
for  the  determination  of  phosphoric  acid,  the  rest  of  the 
manganese  would  be  thrown  down,  as  phosphate,  with  the 
magnesium  ammonium  phosphate,  and  the  results  would  be 
too  high. 

IRON,  MANGANESE,  ALUMINIUM,  AND   PHOSPHORIC 

ACID. 

"When  aluminium  is  also  present,  it  is  not  feasible  to 
precipitate  the  manganese  as  dioxide ;  aluminium  phosphate 
is  always  precipitated  with  it,  even  when  the  dioxide  is 
redissolved  and  reprecipitated.  Neither  citric  nor  tartaric 
acid,  nor  glycerine,  will  prevent  the  precipitation  of  the 
aluminium  phosphate.  The  presence  of  phosphoric  acid, 
together  with  aluminium,  necessitates  the  precipitation  of  the. 
manganese  as  manganous  sulphide.  After  electrolyzing  as 
usual,  the  iron-free  solution  is  poured  into  a  beaker.  The 
manganese  dioxide  which  adheres  to  the  positive  electrode  is 
dissolved  in  hydrochloric  acid,  the  solution  treated  with  pure 
sodium  hydro'xide  in  excess,  and  added  to  the  principal  solu- 

*  If  the  acidifying  is  omitted,  and  the  solution  at  once  precipitated  with 
magnesium  chloride  solution,  crystals  of  hydrogen  potassium  ammonium 
carbonate  separate  with  the  magnesium  ammonium  phosphate,  and  cannot 
be  removed  by  washing  with  dilute  ammonia. 


SEPARATION   OF  THE   METALS.  129 

tion.  Tartaric  acid  is  now  added,  then  ammonia  to  alkaline 
reaction,  and  finally  ammonium  sulphide.  After  standing 
three  or  four  hours,  the  manganese  is  all  separated  as  green 
manganous  sulphide,  which  is  determined  as  usual. 

There  is  little  advantage,  in  this  case,  in  the  use  of  elec- 
trolysis for  the  determination  of  phosphoric  acid ;  it  is  best 
determined  in  a  separate  portion  by  molybdate  solution. 

IRON,  MANGANESE,  AND   SULPHURIC   ACID. 

As  in  the  separation  from  phosphoric  acid,  the  iron  and 
manganese  are  converted  into  double  oxalates  and  electro- 
lyzed.  If  it  is  not  desired  to  determine  the  manganese,  the 
sulphuric  acid  may  be  directly  determined  in  the  filtrate  from 
manganese  dioxide  after  electrolysis,  without  separating  the 
rest  of  the  manganese  with  hydrogen  dioxide  or  hypochlorite  ; 
it  is  only  necessary  to  acidify  with  hydrochloric  acid,  and 
precipitate  with  barium  chloride. 

COPPER   AND   BISMUTH. 

The  metals  cannot  be  separated  from  the  solution  of  the 
double  oxalates ;  both  metals  are  always  deposited.  They 
may  be  separated,  however,  from  nitric  acid  solution.  The 
copper  is  precipitated  as  directed  p.  89 ;  the  bismuth  is  then 
precipitated  from  the  copper- free  solution,  after  removal  of 
the  free  nitric  acid  and  conversion  into  sulphates. 

Edgar  F.  Smith  separates  the  two  metals  by  treating  with 
citric  acid  (3  gm.  to  0.2  gm.  of  the  two  metals),  then  with 
potassium  hydroxide  to  alkaline  reaction,  and  adding  potas- 
sium cyanide  in  slight  excess.  The  solution  must  be  clear 
after  the  addition  of  potassium  cyanide,  otherwise  the  treat- 
ment with  citric  acid  and  potassium  hydroxide  is  repeated. 


130        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

The  solution  is  diluted  to  about  200  cc.,  and  electroljzed  with 
a  current  of  1  cc.  electrolytic  gas  per  minute. 

COPPER   AND    CADMIUM. 

It  is  not  possible  to  separate  copper  and  cadmium  from  the 
solution  of  the  double  oxalates,  nor  from  a  sulphuric  acid 
solution,  as  in  the  latter  case  the  action  of  the  current  is  ex- 
ceedingly slow.  They  may,  however,  be  completely  separated 
by  the  use  of  a  nitric  acid  solution.  As  in  the  separation  from 
bismuth,  the  copper,  is  precipitated  from  this  solution,  the 
nitric  acid  evaporated  off,  and  the  cadmium  converted  into1  the 
sulphate  and  electrolyzed  as  directed  on  p.  94. 

To  separate  these  metals,  Edgar  F.  Smith  treats  with  di- 
sodium  phosphate  so  long  as  a  precipitate  is  formed,  and  dis- 
solves the  precipitate  in  phosphoric  acid.  The  solution  is 
diluted  with  water,  and  subjected  to  the  action  of  a  current  of 
0.2  cc.  electrolytic  gas  per  minute. 

COPPER   AND  LEAD. 

As  already  stated  (p.  96),  lead  separates  from  a  solution 
containing  free  nitric  acid,  at  the  positive  electrode,  as  per- 
oxide. The  separation  of  lead  from  copper  is  complete  and 
causes  no  trouble  if  the  directions  given  on  p.  97  are  closely 
followed. 

COPPER   AND    SILVER. 

A  neutral  or  weakly  acid  solution  of  a  silver  salt  treated 
with  ammonium  oxalate  gives  a  white  precipitate  of  silver 
oxalate  insoluble  in  an  excess  of  the  precipitant.  A  copper 
solution  treated  in  the  same  way  yields  soluble  copper  ammo- 
nium oxalate.  To  separate  the  two  metals,  therefore,  ammo- 
nium oxalate  is  added  to  the  solution  containing  them  both 
till  the  precipitate  of  silver  oxalate  appears  of  a  clear  white ; 


-   '• 
• 


SEPARATION   OF  THE   METALS.  131 

it  is  then  filtered  off,  washed  first  with  ammonium  oxalate, 
ihen  with  pure  water,  and  dissolved  in  potassium  cyanide. 

The  silver  is  deposited  from  this  solution  as  described 
p.  101,  and  the  copper  from  the  filtrate  according  to  p.  88. 

According  to  Edgar  F.  Smith  these  two  metals  may  be 
separated  by  the  use  of  potassium  cyanide.  To  about  0.4  gm. 
of  the  mixed  metals  4.5  gm.  potassium  cyanide  is  added,  the 
solution  diluted  to  about  200  cc.,  and  electrolyzed  with  a  cur- 
rent of  0.1-0.4  cc.  electrolytic  gas  per  minute.  Sivler  alone 
separates,  free  from  copper.  The  precipitation  requires  about 
16  hours. 

COPPER   FROM   ANTIMONY  AND   ARSENIC. 

Copper  can  be  separated  from  these  two  metals  in  oxalate 
or  in  acid  solution  only  when  the  quantity  of  arsenic  or  anti- 
mony is  small,  and  when  the  current  is  allowed  to  act  only 
long  enough  to  reduce  the  copper.  Directions  have  already 
been  given  (p.  91)  as  to  the  procedure  when  small  quantities 
of  antimony  or  arsenic  are  deposited  on  the  copper.  If  more 
than  0.2  gin.  arsenic  is  present,  the  copper  appears  more  or 
less  darkened  at  the  close  of  the  analysis,  so  that  too  high  a 
result  is  obtained  on  weighing.  It  has  been  proposed,  accord- 
ing to  the  directions  on  p.  91,  thereafter  to  ignite  the  dried 
electrode  until  the  arsenic  volatilizes,  dissolve  the  copper  oxide 
that  remains,  and  repeat  the  electrolysis.  This  procedure  gives 
satisfactory  results  when  the  quantity  of  arsenic  is  very  small. 
Experiments  in  the  Aachen  laboratory  on  native  sulphides,  etc., 
rich  in  arsenic  show  that  in  presence  of  much  arsenic  the  pre- 
cipitation of  copper  is  not  complete,  and,  moreover,  that  it  is 
impossible  fully  to  volatilize  a  large  quantity  of  arsenic  by  heat. 
After  the  electrode  is  gently  ignited,  a  residue  insoluble  in 
nitric  acid  is  always  left.  Electrolysis  of  the  solution  obtained 
is,  moreover,  unsatisfactory. 


7NI7ERSIT7 


132         QUANTITATIVE   ANALYSIS   BY   ELECTKOLYSIS. 

Copper  cannot  therefore  be  immediately  determined  from 
a  solution  containing  much  arsenic.  The  determination  of 
copper  in  arseniferous  ores,  etc.,  presents  no  difficulties  if  the 
solution  is  treated  with  bromine ;  all  the  arsenic  volatilizes  as 
arsenous  bromide.  To  prove  the  accuracy  of  this  method, 
metallic  copper  and  metallic  arsenic,  both  obtained  by  elec- 
trolysis, were  dissolved  in  hydrochloric  acid  with  addition  of 
bromine,  and,  according  to  the  amount  of  arsenic  present, 
evaporated  three  or  four  times  on  the  water-bath  with  addi- 
tion of  bromine  water.  To  decompose  the  resulting  bromine 
compound  of  copper,  it  was  heated  with  sulphuric  acid,  and 
the  solution  diluted  after  cooling,  treated  with  the  necessary 
quantity  of  nitric  acid,  and  electrolyzed. 

From  concentrated  sulphides,  arsenic  is  removed  by  repeated 
heating  with  a  few  cc.  of  bromine,  sufficiently  to  allow  the  pre- 
cipitation of  the  copper  free  from  arsenic.  From  roasted  ores 
it  cannot  be  removed  by  bromine  alone,  but  yields  to  repeated 
evaporation  with  solution  of  bromine  in  hydrochloric  acid. 

Arseniferous  residues  are  twice  evaporated  on  the  water- 
bath  with  hydrochloric  acid  and  bromine,  treated  with  a  double 
weight  of  concentrated  sulphuric  acid,  heated,  first  in  the  water- 
bath,  then  in  the  sand-bath,  until  sulphuric  anhydride  fumes 
are  disengaged,  and  after  cooling  treated  with  30  cc.  of  nitric 
acid  sq.  gr.  1.2.  The  solution  is  diluted  with  water  and  electro- 
lyzed. At  the  beginning  of  electrolytic  action  copper  is 
usually  not  precipitated,  but  begins  to  deposit  only  after  most 
of  the  iron  present  is  reduced  from  the  ferric  to  the  ferrous 
state  by  the  current.  With  products  rich  in  lead  it  is  best  to 
filter  the  sulphuric  acid  solution  obtained  as  above,  and  to 
add  the  necessary  quantity  of  nitric  acid  to  the  filtrate. 

According  to  Edgar  F.  Smith,  arsenic-free  copper  is  ob- 
tained, when  the  arsenic  is  present  as  arsenate  in  the  solution, 


SEPARATION   OF   THE   METALS.  133 

by  treating  with  potassium  cyanide  in  excess,  and  using  for 
electrolysis  a  current  of  1.5-5  cc.  electrolytic  gas  per  minute. 

Smith  also  employs  the  foregoing  method  for  the  separa- 
tion of  cadmium  and  silver  from  arsenic.  A  current  of  about 
0.4  cc.  per  minute  is  sufficient  for  this  purpose. 

COPPER   AND   TIN. 

When  a  solution  of  the  two  metals  is  digested  with  sodium 
sulphide,  there  are  formed,  as  is  well  known,  soluble  sodium 
tin  sulphide  and  insoluble  copper  sulphide.  They  are  sepa- 
rated by  filtration,  the  tin  determined  according  to  directions 
given  p.  112,  and  the  copper  in  nitric  acid  solution,  as  above. 

COPPER  FROM  IRON,  COBALT,  NICKEL,  ZINC,  MAN- 
GANESE, CHROMIUM,  ALUMINIUM,  AND  PHOSPHORIC 
ACID. 

The  fact  that  copper  may  be  deposited,  by  a  very  weak 
current  (see  p.  88),  from  a  solution  containing  ammonium 
oxalate  in  excess,  may  be  utilized  for  its  separation  from  the 
above-named  substances.  The  double  oxalate  is  formed  by 
adding  potassium  oxalate  to  a  neutral  or  weakly  acid  solution, 
and  diluting  with  ammonium  oxalate  solution  to  a  volume  of 
170-200  cc.  The  copper  is  precipitated  by  the  use  of  two 
Bunsen  cells  joined  zinc  to  zinc  and  carbon  to  carbon;  and 
the  iron,  cobalt,  etc.,  are  determined  in  the  solution,  after 
removal  of  the  copper,  by  dissolving  2  or  3  grams  more 
ammonium  oxalate,  and  electrolyzing  as  heretofore  directed. 

COPPER     FROM     BARIUM,    STRONTIUM,    CALCIUM, 
POTASSIUM,    SODIUM,    AND    LITHIUM. 

It  is  most  convenient  to  precipitate  from  nitric  acid  solu- 
tion ;  but  care  must  be  taken  that  free  nitric  acid  is  always 
present,  else  the  ammonia  which  is  produced,  uniting  with 


134        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

carbonic  acid,  may  precipitate,  on  the  copper,  carbonates  of 
the  alkaline  earth  metals,  which  cannot  easily  be  removed. 

[BISMUTH  FROM  IRON,  NICKEL,  COBALT,  ZINC,  MAN- 
GANESE,  CADMIUM,  CHROMIUM,  ALUMINIUM,  AND 
URANIUM. 

Smith  and  Knerr*  separate  bismuth  from  the  above- 
named  metals,  by  electrolysis  from  a  sulphate  solution  con- 
taining free  sulphuric  acid.  In  a  large  number  of  experi- 
ments given,  the  free  H2SO4  varied  from  1  cc. :  50  cc.  solution 
to  2  cc.  :  20  cc.  solution,  the  volume  of  solution  from  20  to 
150  cc.,  the  bismuth  from  0.004  to  0.13  gin.,  other  metals 
from  0.005  to  0.04  gm.,  the  strength  of  current  from  1  to  3.5 
<ic.  oxyhydrogen  gas  per  hour,  and  the  time  consumed  from 
thirty  minutes  to  four  hours.  Results  in  all  cases  very 
accurate. — Trans.~\ 

LEAD   AND   CADMIUM. 

The  process  is  the  same  as  in  the  separation  of  lead  and 
copper,  lead  being  separated  as  peroxide  in  nitric  acid  solu- 
tion, and  the  platinum  dish  made  the  positive  electrode 
(p.  97).  To  determine  cadmium  in  the  lead-free  solution, 
the  nitric  acid  is  evaporated  off,  and  the  cadmium  converted 
into  sulphate  and  treated  as  directed  on  p.  94. 

LEAD  AND   BISMUTH. 

The  process  is  precisely  like  the  foregoing. 

LEAD   AND    SILVER, 

A.8  silver  is  fully  precipitated  from  a  nitric  acid  solution 
"as  metal  (p.  102),  and  lead  as  peroxide  at  the  positive  elec- 

*  Am.  Ch.  J.,  8,  207. 


SEPARATION    OF    THE   METALS.  135 

trode  (p.  97),  both  metals  may  be  determined  at  once  in  the 
same  solution. 

LEAD    AND    MERCURY. 

Mercury,  like  silver,  is  deposited  from  a  nitric  acid  solu- 
tion ;  it  can  therefore  be  separated  from  lead  in  the  same 
way. 

LEAD   FROM   IRON,    COBALT,   NICKEL,    ZINC,   CHROMIUM> 
AND     ALUMINIUM. 

The  nitric  acid  solution  is  electrolyzed  as  directed  on  p.  97, 
and  as  large  a  surface  as  possible  furnished  to  the  lead 
dioxide  by  making  the  platinum  dish  the  positive  electrode. 

CADMIUM    AND     ZINC. 

The  separation  depends  on  the  same  principle  as  the 
separation  of  copper  from  other  metals  (see  p.  91).  Eliasberg 
has  investigated  the  subject  in  the  author's  laboratory,  and 
finds  that  the  separation  is  complete  and  satisfactory  when 
the  solution  is  kept  warm  throughout,  and  a  current  of  0.1- 
0.15  cc.  oxyhydrogen  gas  per  minute  is  employed  for  the 
separation  of  the  cadmium.  8  to  10  grams  potassium  oxalate 
are  dissolved  in  the  acid-free  solution,  it  is  heated  with  the 
addition  of  2-3  gms.  ammonium  oxalate,  diluted  to  about 
100  cc.,  and  electrolyzed  as  directed.  The  cadmium  is  com- 
pletely separated  in  six  to  seven  hours  ;  it  separates  as  a 
partly  compact,  partly  crystalline,  deposit  on  the  platinum 
dish. 

The  zinc  is  determined  in  the  solution  as  directed  on 
p.  83. 

A.  Yver  *  recommends  the  use  of  a  solution  of  the  ace- 

*  Bull.  boc.  Chim.  de  Paris,  34,  18. 


136         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS 

rates  or  sulphates  treated  with  an  excess  of  sodium  acetate 
and  a  few  drops  of  acetic  acid  ;  the  electrolysis  to  be  con- 
ducted hot,  using  two  Daniell  cells.  According  to  Eliasberg's 
experiments,  the  action  is  satisfactory  when  the  current  has 
a  strength  of  0.5-0.6  cc.  oxyhydrogen  gas,  and  the  solution 
is  diluted  to  80-90  cc. 

[Smith  and  Knerr*  have  also  investigated  this  process, 
and  obtain  the  best  results  with  a  current  of  not  much  more 
than  0.1  cc.  oxyhydrogen  gas. 

They  also  separate  cadmium  from  zinc  in  a  tartrate 
solution,  containing  2  gms.  sodium  tartrate  and  excess  of 
tartaric  acid,  employing  a  current  generating  0.4-0.5  cc. 
oxyhydrogen  gas  per  minute.  The  cadmium  is  deposited 
rapidly  at  first,  but  the  last  traces  are  difficult  to  remove. 
Duration  of  process,  two  and  one-half  to  four  and  one- 
half  hours.  —  Trans. ~\ 

In  the  laboratory  of  the  Technical  High  School  in  Munich 
the  following  directions  are  given  for  Tver's  method:  To  the 
sulphuric  acid  solution  of  the  two  metals  add  sodium  hydrox- 
ide solution  until  a  permanent  precipitate  is  obtained,  dissolve 
the  precipitate  in  the  smallest  possible  quantity  of  dilute  sul- 
phuric acid,  dilute  the  solution  to  about  TO  cc.  and  reduce  the 
cadmium  with  a  current  of  N.  D.100  —  0.07  ampere  (see  p.  63). 
"When  the  greater  part  of  the  metal  is  precipitated  neutralize 
the  free  sulphuric  acid  with  sodium  hydroxide,  add  3  gm. 
sodium  acetate,  heat  to  about  45°,  and  subject  to  the  action  of 
a  current  of  N.  D.100  =  0.3  ampere.  The  latter  direction  as- 
sumes that  the  electromotive  force  is  not  over  3.6  volts ;  if 
more,  it  is  to  be  reduced  to  about  2.4  volts. 

Edgar  F.  Smith  and  Lee  K.  Frankel  recommend  the  sepa- 
ration of  the  two  metals  by  precipitating  the  cadmium  from 

*  Am.  Ch.  J.,  8,210. 


SEPARATION   OF   THE   METALS.  137 

the  solution  of  the  double  cyanides.  The  solution  is  treated 
with  potassium  cyanide  in  such  excess  that  4.5  gm.  are  present 
to  0.4  gin.  of  the  metals,  diluted,  and  a  current  of  0.3  cc.  elec- 
trolytic gas  used  for  the  electrolysis.  Precipitation  is  com- 
plete in  18  to  24  hours.  Zinc  is  precipitated  by  this  weak 
current  only  when  all  the  cyanide  is  decomposed.  It  is  deter- 
mined in  the  cadmium-free  solution,  as  heretofore  directed 
<p.  83). 

CADMIUM   FROM   NICKEL   AND    COBALT 

The  separation  is  made,  as  above  directed,  from  potassium 
cyanide  solution,  with  a  current  of  about  0.4  cc.  electrolytic  gas. 

CADMIUM     AND     BISMUTH     FROM     MANGANESE, 
CHROMIUM,     AND     ALUMINIUM. 

Cadmium  and  bismuth  are  precipitated  according  to  pp.  94 
and  91,  and  manganese,  chromium,  and  aluminium  separated 
according  to  previous  directions. 

MERCURY     AND     SILVER. 

Both  metals  are  deposited,  as  stated  p.  102,  from  a  nitric 
acid  solution.  The  sum  of  mercury  and  silver  is  deter- 
mined, the  former  then  driven  off  by  heat,  and  the  silver 
weighed. 

MERCURY     AND     COPPER. 

According  to  Edgar  F.  Smith  and  Lee  K.  Frankel,  these 
metals  may  be  separated  by  a  process  analogous  to  that  given 
for  zinc  and  cadmium  on  p.  136.  If  the  solution  of  the  double 
cyanides  is  electrolyzed  with  a  current  of  2  cc.  electrolytic  gas 
per  minute,  only  mercury  is  precipitated.  The  precipitate 
must  be  washed  with  water  only,  because,  as  heretofore  stated, 


138         QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

alcohol  washes  away  a  portion  of  the  metal  when  precipitated 
from  alkaline  solution.  Results  are  always  satisfactory  when 
less  copper  than  mercury  is  present. 

MERCURY     FROM     ARSENIC     AND     PALLADIUM. 

The  behavior  of  the  double  cyanides  under  the  action  of 
the  electric  current,  so  largely  available  for  separations,  may 
be  used  to  separate  mercury  from  these  two  metals.  In  the 
separation  from  arsenic  the  solution,  treated  with  potassium 
cyanide  in  excess,  is  exposed  to  a  current  of  0.3  cc.  elec- 
trolytic gas,  in  the  separation  from  palladium  to  one  of 
0.1-0.2  cc. 

MERCURY     FROM     IRON,     COBALT,     NICKEL,     ZINC, 
MANGANESE,     CHROMIUM,     AND     ALUMINIUM. 

The  separation  depends  on  the  fact  that  mercury  is  pre- 
cipitated from  a  solution  acidified  with  nitric  acid  (p.  102),. 
from  which  the  other  metals  are  not  removed. 

ANTIMONY     AND     TIN. 

The  separation  of  antimony  from  tin,  which,  as  is  well 
known,  is  difficult,  and  gives  uncertain  results,  by  the  ordi- 
nary gravimetric  methods,  may  be  accomplished  by  electroly- 
sis with  ease  and  accuracy.  Antimony  may  be  completely 
precipitated,  in  the  presence  of  tin,  from  a  concentrated  solu- 
tion of  sodium  sulphide,  to  which  is  added  a  certain  amount 
of  sodium  hydroxide  ;  the  strength  of  current  used  corresponds 
to  1.5-2  cc.  oxy hydrogen  gas  per  minute. 

The  crystallized  sodium  monosulphide  of  commerce,  aside 
from  the  fact  that  its  purity  is  otherwise  uncertain,  is  not 
pure  monosulphide,  but  is  a  mixture  of  several  sulphides 
with  varying  amounts  of  sodium  hydroxide.  This  explains 


SEPARATION   OF  THE   METALS.  139> 

the  large  amount  of  alumina  which  it  always  contains.  If, 
therefore,  commercial  sodium  sulphide  is  to  be  used,  it  must 
first  be  dissolved,  and  the  solution,  with  exclusion  of  air, 
completely  saturated  with  pure  hydrogen  sulphide  gas.  It  is 
then  filtered  from  the  precipitated  impurities,  and  evaporated 
in  a  large  platinum  or  porcelain  dish.  The  further  treatment 
is  given  in  full  in  the  chapter  on  reagents.  It  is  preferable, 
however,  as  the  condition  of  the  sodium  sulphide  solution  is 
of  great  importance  to  the  success  of  the  process,  to  prepare 
the  solution  as  directed  in  the  chapter  referred  to. 

The  process  of  separation  is  as  follows :  A  mixture  of 
the  pure  sulphides,*  or  the  residue  obtained  by  evaporating 
a  solution  of  the  two  metals,  is  treated  in  a  platinum  dish 
with  about  60  cc.  of  a  sodium  sulphide  solution  of  sp.gr. 
1.22-1.225,  and  enough  concentrated  solution  of  sodium  hy- 
droxide to  furnish  1  gm.  NaOH.  If  solution  does  not  take 
place  at  once,  it  is  hastened  by  heating  over  a  low  flame, 
the  watch-glass  covering  the  dish  is  rinsed  with  10-15  cc. 
water,  and  the  solution  is  allowed  to  cool  thoroughly.  It 
is  then  submitted  to  the  action  of  a  current  yielding 
1.5-2  cc.  per  minute,  which  may  be  produced  by  several 
Meidinger  cells,  or  by  reducing  to  the  required  strength 
the  current  from  Bunsen  cells,  dynamo,  or  accumulators. 
It  is  best  to  leave  the  current  at  work  during  the  night ; 
the  separation  is  completed  in  twelve  hours,  the  antimony 
appearing  as  a  brilliant  coating,  adhering  closely  to  the  dish. 
When  the  action  begins,  the  whole  surface  of  the  dish,  which 
is  in  contact  with  the  solution,  becomes  quickly  covered 
with  a  dark  coating  of  antimony  which  soon  takes  on  a 
brilliant  metallic  appearance. 


*  The  solution  of  a  mixture  of  the  metallic  sulphides  and  sulphur  in 
sodium  sulphide  is  to  be  treated  like  a  solution  of  polysulphides  (see  p.  108)- 


140        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

In  the  earlier  part  of  the  process,  the  whole  solution 
appears  to  be  filled  with  small  gas  bubbles  which  rise  slowly, 
break  at  the  surface,  and  cover  the  watch-glass  with  minute 
portions  of  the  solution.  In  the  course  of  two  hours,  the 
disengagement  of  gas  is  ended,  and  the  solution  is  completely 
clear.  To  avoid  loss,  it  is  best,  at  this  time,  repeatedly  to 
wash  the  under  surface  of  the  watch-glass  with  a  drop  of 
water  which  finally  runs  down  the  positive  electrode.  When 
the  current  has  been  in  action  for  twelve  hours,  it  is  stopped, 
the  solution  is  poured  into  a  second  tared  platinum  dish, 
and  the  deposited  metal  is  washed  two  or  three  times  with 
about  10  cc.  of  water.  The  antimony  is  treated  according  to 
the  directions  already  given,  and  weighed. 

As  tin  cannot  be  reduced  from  a  sodium  sulphide  solu- 
tion, but  can  be  completely  precipitated  from  solution  in 
ammonium  sulphide  (as  stated  on  p.  110),  the  sodium  sulphide, 
after  the  separation  of  antimony,  must  be  converted  into 
ammonium  sulphide  according  to  the  directions  given  on 
p.  112. 

If  the  two  metals  are  to  be  determined  in  the  yellow 
solution  of  polysulphides  of  the  alkalies,  the  solution  is 
decolorized  with  ammoniacal  hydrogen  peroxide  (see  Anti- 
mony, p.  112),  and  evaporated  nearly  to  dryness ;  about 
60  cc.  sodium  sulphide  solution  and  the  necessary  amount 
of  sodium  hydroxide  are  then  added,  and  the  process  carried 
on  as  above  directed. 


ANTIMONY    AND     ARSENIC. 

In  an  alkaline  solution,  arsenious  acid  is  oxidized  to 
arsenic  acid  by  the  galvanic  current.  If,  however,  a  solution 
containing  antimony  and  arsenious  acid  is  electrolyzed,  a 
mixture  of  antimony  with  arsenic  is  deposited.  The  action 


SEPARATION    OF   THE   METALS.  141 

is  different  if  the  arsenic  is  present  in  the  solution  as  arsenic 
acid ;  in  the  presence  of  a  free  alkali,  the  antimony  alone 
is  deposited  from  a  concentrated  sodium  sulphide  solution. 
The  arsenic,  therefore,  if  present  as  arsenious  acid,  must  be 
oxidized  to  arsenic  acid  before  the  metals  can  be  separated. 
It  is  heated  with  concentrated  nitric  acid  or  aqua  regia,  the 
acid  completely  removed  by  evaporation  on  the  water-bath, 
the  residue  treated  with  50-60  cc.  sodium  sulphide,  sp.  gr. 
1.22-1. 22& ;  a  concentrated  solution  of  sodium  hydroxide 
containing  about  1  gm.  NaOH  added,  and  the  solution  sub- 
mitted to  the  action  of  a  current  yielding  1.5-2  cc.  oxy- 
hydrogen  gas  per  minute.  The  separation  is  conducted 
precisely  like  that  of  antimony  from  tin. 

If  antimony  and  arsenic  are  to  be  determined  in  a  solution 
of  polysulphides  of  the  alkalies,  the  solution  is  treated  as 
described  on  p.  108.  To  determine  arsenic,  the  antimony-free 
solution  is  acidified  with  dilute  sulphuric  acid,  heated  in 
the  water-bath  to  remove  hydrogen  sulphide,  filtered,  and  the 
precipitate  dissolved  in  hydrochloric  acid  with  the  addition 
of  potassium  chlorate.  This  solution  is  treated  with  ammonia 
in  excess,  and  the  arsenic  acid  precipitated  as  magnesium 
ammonium  arsenate  with  magnesia  mixture. 

The  precipitate  may  be  dried,  at  110°,  on  a  weighed  filter, 
and  weighed,  or  converted  into  magnesium  pyro-arsenate  by 
careful  ignition  in  a  porcelain  crucible. 

ARSENIC,    ANTIMONY,    AND    TIN. 

If  arsenic  is  present  as  arsenic  acid,  antimony  alone  is 
precipitated  from  a  concentrated  alkaline  solution  of  the 
three  metals  in  sodium  sulphide  ;  tin  and  arsenic  remain  in 
solution.  The  arsenic  is  converted  into  arsenic  acid,  and  the 
antimony  precipitated,  exactly  as  heretofore  described. 


142         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS 

For  the  separation  of  tin  from  arsenic,  the  solution  poured 
off  from  the  antimony  is  treated  with  dilute  sulphuric  or 
hydrochloric  acid  to  decompose  the  sulpho-salts,  the  mixture 
of  arsenic  and  tin  sulphides  and  sulphur  is  filtered  off,  and 
oxidized  with  hydrochloric  acid  and  potassium  chlorate, 
and  the  arsenic  separated  as  above  described.  To  determine 
the  tin,  the  solution  freed  from  arsenic  is  saturated  with 
hydrogen  sulphide,  filtered,  and  the  tin  sulphide  dissolved  in 
ammonium  sulphide.  The  tin  is  determined  electrolytically 
as  directed  p.  111. 

In  the  analysis  of  a  substance  which  contains  arsenic, 
antimony,  and  tin,  the  arsenic  may  also  be  first  eliminated 
according  to  the  method  of  E.  Fischer-Hufschmidt  simplified 
by  R.  Ludwig  and  the  author,*  and  antimony  and  tin  sepa- 
rated in  the  arsenic-free  solution. 

If  the  sulphides  of  the  metals  are  to  be  separated,  they 
are  oxidized  with  concentrated  hydrochloric  acid  and  potas- 
sium chlorate,  and  the  acid  evaporated  on  the  water-bath. 
The  residue  is  washed  with  fuming  hydrochloric  acid  into 
a  flask  of  500-600  cc.  capacity,!  treated  with  20-25  cc.  of  a 
saturated  solution  of  ferrous  chloride,  or  better,  with  about 
25  gm.  of  ammonium  ferrous  sulphate  [FeSO4  -f-  (NH4)2SO4 
+  6H2O]  and  fuming  hydrochloric  acid  added  till  the  volume 
is  150  to  200  cc.  A  strong  current  of  hydrochloric  acid  gas 
is  now  passed  into  the  solution,  and  kept  up  for  at  least  half 
an  hour  after  the  solution  seems  fully  saturated.  Then  the 
solution  is  reduced  to  about  50  cc.  by  distilling  off  the  liquid, 
without  a  condenser,  in  a  stream  of  hydrogen  chloride  gas. 
A  flask  of  about  1  litre  capacity,  containing  400-500  cc. 


*  Ber.  d.  ch.  Ges.,  18, 1110. 

t  A  convenient  apparatus  is  illustrated  in  the  author's  "  Handbuch  de* 
Quantitative  Analyse,"  4th  edition,  p.  78. 


SEPARATION   OF   THE   METALS.  143 

water,  is  used  as  a  receiver.  If  the  flask  is  well  cooled 
during  the  distillation,  not  a  trace  of  arsenic  passes  over  into 
a  second  receiver,  even  when  as  much  as  0.5  gm.,  reckoned 
as  As2O3,  is  present. 

The  arsenic  in  the  distillate  may  either  be  saturated  with 
sodium  carbonate,  and  titrated  with  iodine  solution,  or  pre- 
cipitated as  As2O3  with  hydrogen  sulphide,  and  determined 
as  such  on  a  weighed  filter,  or  the  arsenic  calculated  from 
the  amount  of  sulphur  in  the  precipitate.  The  process,  in  the 
latter  case,  is  as  follows :  The  distillate  is  mixed  with  twice 
its  volume  of  water,  air  expelled  by  a  strong  current  of 
carbon  dioxide,  and  the  arsenic  precipitated  by  passing  in 
pure  hydrogen  sulphide  gas.  The  excess  of  hydrogen  sul- 
phide is  removed  by  passing  a  strong  current  of  carbon 
dioxide  till  lead  acetate  paper  is  not  colored  by  the  escaping 
gases.  The  arsenic  sulphide  is  allowed  to  subside,  and  the 
clear  solution  siphoned  off.  The  remaining  strongly  acid 
solution  is  saturated  with  ammonia,  which  dissolves  the 
arsenic  sulphide  ;  the  solution  is  then  boiled  with  an  excess 
of  hydrogen  peroxide  free  from  sulphuric  acid.  The  solution 
is  then  acidified  with  hydrochloric  acid,  and  the  sulphuric 
acid  produced  by  the  action  of  the  hydrogen  peroxide  deter- 
mined as  barium  sulphate  in  the  usual  way. 

To  determine  the  antimony  and  tin,  the  strong  acid  solu- 
tion in  the  flask,  which  contains  the  iron,  is  diluted  with 
three  times  its  volume  of  water.  Antimony  and  tin  are 
precipitated  with  hydrogen  sulphide.  After  the  precipitate 
has  subsided,  the  clear  solution  is  poured  on  a  filter,  the 
precipitate  washed  several  times  by  decantation,  and  after- 
wards, on  the  filter,  with  hot  water,  till  free  from  hydro- 
chloric acid.  Portions  of  the  sulphides  often  adhere  to  the 
walls  of  the  flask  in  which  the  precipitation  took  place. 
These  are  washed  out  with  concentrated  sodium  sulphide 


144         QUANTITATIVE  ANALYSIS   BY    ELECTKOLYSIS. 

solution,  and  the  solution  is  poured  on  the  filter  containing- 
the  sulphides.  The  filtrate  is  collected  in  a  tared  platinum 
dish.  The  filter,  on  which  some  iron  sulphide  always 
remains  after  the  solution  of  the  antimony  and  tin  sulphides, 
is  washed  with  sodium  sulphide  solution,  the  necessary 
amount  of  sodium  hydroxide  is  added  to  the  filtrate,  and 
the  antimony  and  tin  are  separated  electrolytically  as  already 
directed. 

TIN    AND    PHOSPHORIC    ACID. 

In  the  determination  of  metals,  in  the  presence  of  phos- 
phoric acid,  the  latter  is  often  removed  as  tin  phosphate. 
The  phosphoric  acid  is  then  usually  determined  in  a  separate 
portion,  as  its  determination  in  the  tin  precipitate  is  too 
difficult  and  slow  a  process.  The  precipitate  of  tin  oxide 
and  tin  phosphate  may,  however,  be  dissolved  by  digestion 
with  ammonium  sulphide,  the  solution  diluted,  the  tin  pre- 
cipitated by  electrolysis,  and  the  phosphoric  acid  determined 
as  usual. 

PLATINUM    AND    IRIDIUM. 

As  stated  on  p.  105,  platinum  can  be  separated  from  a 
hydrochloric  acid  solution,  in  a  compact  condition,  by  a  very 
weak  current.  This  fact  may  be  utilized  for  its  separation 
from  iridium.  If  the  current  from  two  or  three  Meidinger 
cells,  or  a  single  Bunsen  cell,  is  passed  through  an  acidified 
solution  of  platinum  and  iridium,  the  platinum  is  separated 
without  the  least  trace  of  iridium. 


SEPARATION    OF    GOLD    FROM    OTHER    METALS. 

Edgar  F.  Smith  has  been  often  quoted  as  having  given 
much  study  to  the  behavior  of  the  metallic  cyanides  under 


SEPARATION   OF  THE   METALS.  145 

the  electric  current.  He  bases  thereon  a  method  for  the  sepa- 
ration of  gold  from  palladium,  copper,  nickel,  zinc,  and  plati- 
num. The  conditions  are  the  same  as  those  already  repeatedly 
given.  When  the  solution  of  about  150  cc.  contains  about  3 
gm.  potassium  cyanide,  a  current  yielding  0.5-1  cc.  electro- 
lytic gas  per  minute  precipitates  gold  free  from  the  other 
metals. 

This  method  serves  also  to  separate  silver  or  mercury  from 
platinum.  The  silver  or  mercury  is  thrown  down  entirely 
free  from  platinum. 

POTASSIUM    AND    SODIUM. 

The  ordinary  method  of  determining  potassium  and  so- 
dium in  the  same  solution  is  to  weigh  the  mixed  chlorides, 
and  the  potassium  as  platinchloride ;  the  sodium  is  thus  deter- 
mined by  difference.  The  errors  of  the  work,  therefore,  all 
fall  on  the  sodium.  The  potassium  may  be  determined,  as 
already  directed  (p.  113),  by  precipitating  as  potassium  platin- 
chloride, and  determining  the  platinum  in  the  latter  by  elec- 
trolysis. To  determine  the  sodium  directly,  the  filtrate  from 
the  potassium  pLitinchloride  is  evaporated  on  the  water-bath  to 
remove  alcohol,  the  residue  dissolved  in  water  with  the  ad- 
dition of  a  little  hydrochloric  acid,  and  the  platinum  removed 
by  electrolysis.  The  sodium  chloride  in  the  solution  poured 
off  from  the  platinum  is  determined  by  evaporating  to  dry- 
ness,  and  weighing  the  residue. 

SODIUM    AND    AMMONIA. 

The  direct  determination  of  both  is  accomplished  as  with 
potassium  and  sodium  ;  the  ammonia  is  precipitated  as  ammo- 
nium platinchloride,  and  the  process  conducted  as  described 
above. 


PAET    II. -SPECIAL   PART. 


ALLOY    OF    COPPER    AND    ZINC    (LEAD,    IRON). 
Brass. 

FOE  the  separation  of  the  copper  from  the  other  metals,  it 
Is  necessary  to  precipitate  it  from  acid  solution.  A  nitric  or 
sulphuric  acid  solution  may  be  used.  The  use  of  a  solution 
containing  free  nitric  acid  has  the  disadvantage,  that  if  the 
action  of  the  current  is  continued  after  all  the  copper  is 
precipitated,  more  or  less  zinc  is  carried  down  with  the 
copper.  The  presence  of  nitric  acid  or  a  nitrate  also  hinders 
the  electrolytic  separation  of  the  zinc.  If  this  acid  is  used, 
therefore,  the  solution,  after  removal  of  the  copper,  must  be 
repeatedly  evaporated  to  dryness  with  hydrochloric  acid  to 
convert  the  nitrates  into  chlorides. 

For  the  analysis  of  the  alloy,  0.1-0.2  gm.  is  dissolved  in 
as  little  nitric  acid  as  possible,  evaporated  to  dryness  in  the 
water-bath,  the  residue  dissolved  in  150-200  cc.  water,  and 
15-20  cc.  of  nitric  acid  (sp.  gr.,  1.21)  added.  From  this 
solution,  the  copper  is  precipitated  by  electrolysis  (p.  89). 
The  current  is  continued  as  long  as  a  drop  of  the  solution 
gives  a  blue  color  with  ammonia. 

In  order  to  separate  copper  from  the  other  metals  in 
sulphuric  acid  solution,  the  nitric  acid  solution  of  the  alloy 
is  evaporated  on  the  water-bath,  with  addition  of  about  5  cc. 

147 


148  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

dilute  sulphuric  acid,  till  the  residue  no  longer  smells  of 
nitric  acid,  dissolved  in  150-200  cc.  water,  and  treated  as. 
before.  When  the  reduction  from  either  nitric  or  sulphuric 
acid  solution  is  complete,  the  precipitate  of  metallic  copper 
is  washed  free  from  acid  without  interrupting  the  current 
(p.  89),  the  contents  of  the  dish  siphoned  off,  and  the  copper, 
after  washing  with  "absolute  alcohol,  determined  as  already 
described  (p.  89).  To  determine  the  zinc  in  the  solution,  it 
is  concentrated  to  about  100  cc.,  the  free  sulphuric  acid  very 
nearly  neutralized  with  ammonia,  the  zinc  converted  into  the 
double  oxalate,  and  electrolyzed.  The  process  is  exactly  as- 
described  on  p.  83. 

As  a  rule,  brass  contains  small  quantities  of  lead  and  iron. 
The  presence  of  the  former  is  shown,  during  the  electrolysis 
of  the  copper,  by  the  appearance,  on  the  positive  electrode, 
of  a  slight  brown  coating  of  lead  peroxide.  The  positive 
electrode  is  then  washed  as  usual,  dried,  and  its  increase  in 
weight  determined  (p.  97). 

If  iron  is  present,  it  is  reduced  with  the  zinc.  It  is- 
determined  according  to  directions  on  p.  116. 

ALLOY    OP    COPPER    AND     SILVER. 
Silver  Coin. 

The  alloy  is  analyzed  by  dissolving  0.1-0.2  gm.  in  dilute 
nitric  acid,  evaporating  off  the  acid  in  the  water-bath,  dis- 
solving the  residue  in  water,  and  treating  the  solution  accord- 
ing to  directions  on  p.  130. 


SOLDEE.  149 

ALLOY    OF    TIN    AND    LEAD. 
Solder. 

A  small  quantity  of  the  alloy  is  digested  with  nitric  acid 
till  the  tin  is  entirely  converted  into  oxide,  the  excess  of 
nitric  acid  evaporated,  the  solution  diluted  with  water,  and 
the  tin  oxide  filtered  off.  The  precipitate,  after  washing 
with  water  containing  nitric  acid,  is  dissolved  in  hot  concen- 
trated hydrochloric  acid,  evaporated  on  the  water-bath,  and 
the  aqueous  solution  of  the  residue  converted  into  the  double 
acid  ammonium  oxalate  (see  Tin,  p.  111).  On  submitting  the 
solution  to  electrolysis,  the  small  quantity  of  lead  which  was 
precipitated  with  the  tin  oxide  is  deposited  as  dioxide  on  the 
positive  electrode.  When  the  reduction  is  ended,  both  elec- 
trodes are  therefore  weighed.  The  lead  in  the  nitric  acid 
;solution  is  determined  as  directed  on  p.  97. 

ALLOY    OP    LEAD     AND     BISMUTH. 

The  alloy  is  digested  with  nitric  acid  till  completely  dis- 
solved, the  excess  of  acid  evaporated  off,  and  the  lead  precipi- 
tated as  peroxide  (p.  97). 

The  bismuth  in  the  solution,  after  evaporation  of  the 
nitric  acid,  is  converted  into  bismuth  ammonium  oxalate,  and 
electrolyzed  as  directed  p.  91. 

ALLOY     OP    LEAD     AND    ZINC. 

The  lead  may  be  separated  either  as  peroxide  by  elec- 
trolysis, or  as  sulphate  by  evaporation  with  dilute  sulphuric 
.acid.  In  the  former  case,  the  lead-free  solution  is  evaporated 
to  dryness  with  addition  of  hydrochloric  acid,  and  the  zinc 
finally  determined  as  directed  p.  S3.  If  the  lead  has  been 
determined  as  sulphate,  the  alcohol  is  first  removed,  the 


150         QUANTITATIVE    ANALYSIS   BY   ELECTROLYSIS. 

solution  neutralized  with  ammonia,  and  the  zinc  determined 
as  before. 

ALLOY    OP    BISMUTH    AND    COPPER. 

The  alloy  is  dissolved  in  nitric  acid,  diluted  with  water,, 
and  electrolyzed  as  directed  p.  89  ;  the  copper,  under  the 
action  of  the  prescribed  current,  is  deposited  perfectly  free 
from  bismuth.  To  determine  the  latter,  the  solution  is 
evaporated  to  dryness  in  the  water-bath  to  remove  nitric 
acid,  and  the  residue  treated  as  directed  p.  91. 


ALLOY    OF    COPPER    AND    TIN. 
Bronze. 

The  alloy  is  oxidized  with  aqua  regia,  evaporated  to- 
dryness,  and  the  residue  digested  with  a  concentrated  solu- 
tion of  sodium  sulphide.  The  copper  sulphide  which  remains 
undissolved  is  filtered  off,  washed  thoroughly  first  with 
sodium  sulphide,  then  with  hydrogen  sulphide  solution,  dis- 
solved in  nitric  acid,  and  electrolyzed  as  directed  p.  89.  The 
tin  is  determined  as  directed  p.  111. 

Results  close  enough  for  technical  analysis  are  obtained  by 
oxidizing  the  alloy  with  nitric  acid,  filtering  off  the  tin  oxide, 
dissolving  it,  after  washing,  in  oxalic  acid,  adding  acid  am- 
monium oxalate,  and  precipitating  the  tin,  as  directed  p.  Ill* 
The  copper  is  determined,  as  before,  in  the  filtrate  from  the  tin. 
oxide. 


PHOSPHOR-BRONZE.  151 

ALLOY    OP    COPPER,    TIN,    ZINC,    AND    PHOSPHORUS. 
Phosphor-Bronze. 

When  the  alloy  is  digested  with  concentrated  nitric  acid 
to  complete  oxidation,  a  precipitate  remains,  which  consists 
of  a  mixture  of  tin  oxide  and  tin  phosphate,  with  small 
quantities  of  copper  oxide.  It  is  filtered  off,  washed  with 
water  containing  nitric  acid,  and  heated  with  a  concentrated 
solution  of  sodium  sulphide.  The  residue  of  copper  sul- 
phide is  dissolved  in  nitric  acid,  and  added  to  the  principal 
solution. 

The  tin  is  determined  by  converting  the  sodium  sulphide 
into  ammonium  sulphide,  and  electrolyzing  as  directed  p.  111. 
The  phosphoric  acid  is  determined  in  the  filtrate  in  the  usual 
manner. 

The  nitric  acid  solution  contains  the  copper  and  zinc. 
They  are  separated  according  to  directions  for  the  analysis  of 
brass  (p.  147). 

ALLOY    OP    COPPER,    TIN,    ZINC,    MANGANESE,    AND 
PHOSPHORUS. 

Manganese   Phosphor-Bronze. 

The  process  is  as  already  described  ;  the  manganese 
remains  with  the  zinc,  and  is  finally  separated  as  directed 
p.  123 

ALLOY    OF    NICKEL    AND     COPPER. 
Nickel  Coin. 

The  analysis  of  this  alloy  is  very  simple.  It  is  dissolved 
in  nitric  or  sulphuric  acid,  and  the  copper  precipitated 


152         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

according  to  the  directions  given  on  p.  89.  If  the  copper  is 
precipitated  from  nitric  acid  solution,  the  acid  is  removed  by 
evaporation,  the  nickel  brought  into  solution  as  chloride,  and 
the  process  conducted  as  directed  p.  82. 

If  a  sulphuric  acid  solution  is  used,  the  acid  is  neutralized 
with  ammonia,  and  the  nickel  determined  as  before. 

ALLOY    OP    COPPER,    ZINC,    AND    NICKEL. 
German  Silver. 

The  alloy  is  dissolved  in  nitric  acid,  and  the  copper 
separated  as  directed  p.  89.  The  copper-free  solution  is 
evaporated  in  the  water-bath,  the  nitrates  converted  into 
chlorides  by  heating  and  evaporation  with  hydrochloric  acid, 
the  residue  dissolved  in  water,  with  the  addition  of  hydro- 
chloric acid,  and  diluted  till  1  gin.  of  the  two  oxides  corre- 
sponds to  50  cc.  The  solution  is  nearly  neutralized  by 
adding  sodium  carbonate  till  a  slight  permanent  precipitate 
is  formed,  which  is  redissolved  in  a  few  drops  of  hydrochloric 
acid.  The  zinc  is  precipitated  by  passing  hydrogen  sulphide 
into  the  cold  solution  as  long  as  a  precipitate  is  formed, 
adding  a  few  drops  of  sodium  acetate,  and  allowing  the 
precipitate  to  settle.  The  zinc  sulphide  is  washed  with 
hydrogen  sulphide  water  to  which  a  little  ammonium  nitrate 
is  added,  dissolved  in  concentrated  hydrochloric  acid,  the 
acid  evaporated  off,  and  the  residue  treated  for  determination 
of  zinc  as  directed  p.  83. 

The  filtrate  is  evaporated  to  dryness,  and  the  nickel 
determined  in  the  residue  as  directed  p.  82. 


WOOD'S  METAL.  153 

ALLOY   OP  TIN,  LEAD,   BISMUTH,   AND   CADMIUM. 

Wood's  Metal. 

The  alloy  is  treated  with  nitric  acid,  whereby  the  tin 
Temains  undissolved  as  oxide,  contaminated  with  small  por- 
tions of  the  oxides  of  lead  and  bismuth.  This  is  filtered  off, 
washed  with  water  containing  nitric  acid,  and  dissolved  in 
sodium  sulphide.  The  tin  is  determined  as  directed  p.  111. 

The  insoluble  sulphides  of  bismuth  and  lead  are  dissolved 
in  nitric  acid,  and  the  solution  added  to  the  principal  solution. 
The  lead  is  determined  as  peroxide  (p.  97),  or  as  sulphate, 
observing  the  well-known  precautions. 

The  bismuth  and  cadmium  are  separated  as  follows  :  The 
solution  is  evaporated  to  remove  nitric  acid,  the  residue 
dissolved  in  the  least  possible  amount  of  hydrochloric  acid, 
and  the  bismuth  precipitated  as  oxy chloride  by  the  addition 
of  much  water.  The  precipitate  is  filtered  off,  washed, 
dissolved  in  a  little  dilute  hydrochloric  acid,  and  the  bismuth 
precipitated  as  directed  p.  91.  The  filtrate  from  the  bis- 
muth oxychloride  is  evaporated  to  dryness,  and  the  cadmium 
determined  in  the  residue  as  directed  p.  94. 

ALLOY    OP    TIN,    LEAD,    BISMUTH,    AND     MERCURY. 

The  tin  is  separated  from  the  other  metals  by  oxidation 
with  nitric  acid,  and  treated  as  before.  The  mercury  can 
now  be  precipitated  from  the  acid  solution  (p.  102),  and  also  a 
portion  of  the  lead  as  peroxide  at  the  positive  electrode.  To 
remove  the  whole  of  the  lead,  the  mercury-free  solution  is 
again  electrolyzed,  using  the  platinum  dish  as  the  positive 
electrode.  The  lead-free  solution  is  evaporated  to  dryness, 
and  the  bismuth  determined  as  directed  p.  91.  The  lead 


154         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

may  also  be  separated  from  the  tin-free  solution  as  sulphate,, 
the  mercury  then  separated,  by  electrolysis,  from  the  acid 
solution,  and  the  bismuth  finally  determined  as  before. 

ALLOY    OF    LEAD    AND    ANTIMONY. 
Hard  Lead.     Type  Metal. 

The  two  metals  may  be  separated,  either  by  oxidizing 
with  nitric  acid,  evaporating  to  dry  ness,  and  digesting  the 
residue  with  sodium  sulphide,  or  by  heating  the  finely  divided 
alloy  with  ten  times  its  weight  of  anhydrous  sodium  thio- 
sulphate  in  a  covered  porcelain  crucible,  over  a  very  low 
flame,  till  the  mixture  is  sintered  together,  and  extracting 
with  water.  In  either  case,  lead  sulphide  remains  undis- 
solved,  and  is  filtered  off,  and  washed  first  with  sodium 
sulphide,  and  then  with  hydrogen  sulphide,  solution.  It 
may  be  determined  directly  as  sulphide,  or  as  directed  p.  97. 

The  antimony  is  determined,  in  the  filtrate  from  lead 
sulphide,  exactly  as  directed  p.  107. 

ALLOY    OP    ANTIMONY    AND    TIN. 

The  method  of  analysis  has  been  already  given  on  p.  138.. 
The  alloy  is  oxidized  with  nitric  acid,  and  the  residue,  after 
evaporation,  dissolved  in  a  concentrated  solution  of  sodium 
sulphide,  sodium  hydroxide  added,  and  the  process  followed 
throughout  as  given  on  p.  139 

ALLOY    OP    ANTIMONY    AND     ARSENIC. 

It  has  already  been  stated  (p.  141)  that  the  two  metals 
can  be  separated  under  conditions  similar  to  those  in  the 
separation  of  antimony  from  tin  ;  the  method  requires  the 
arsenic  .to  be  oxidized  to  arsenic  acid.  The  alloy  is  digested 


ALLOY    OF    ANTIMONY,    TIN,    AND    ARSENIC.  155 

with  aqua  regia,  the  acid  removed  by  evaporation,  the  residue 
dissolved  in  concentrated  sodium  sulphide,  sodium  hydroxide 
added,  and  the  directions  given  on  p.  139  followed  throughout. 

ALLOY     OF    ANTIMONY,    TIN,    AND    ARSENIC. 

When  this  alloy  is  oxidized  with  aqua  regia,  and  a  solu- 
tion in  sodium  sulphide  prepared  as  above,  antimony  alone  is 
electrolytically  deposited  in  presence  of  tin.  The  method  is, 
described  on  p.  142. 

SPATHIC    IRON    ORE. 

Constituents :   Ferrous  Carbonate,  with  Manganese,  Calcium, 
and  Magnesium  Carbonates   (Gangue). 

All  the  constituents  of  the  mineral  may  be  determined  in 
the  same  solution.     About   0.5  gm.  of  the  dry  mineral  is 
dissolved  in  a  porcelain  dish,  in  the  least  possible  amount  of 
hydrochloric  acid,  the  acid  removed  by  evaporation,  and  the 
residue  taken  up  with  water  to  which  a  little  hydrochloric 
acid  is  added.     If  insoluble  gangue  is  present,  this  is  filtered 
off,  washed  with  water,  and  weighed.     The  metals  are  con- 
verted into  oxalates  by  treatment  with  potassium  and  ammo- 
nium oxalate,  and  the  insoluble  residue  of  calcium  oxalate 
filtered  off,  and  washed  with  hot  water.     If  manganese  is  pres- 
ent, the  calcium  oxalate  always  carries  down  some  manganese 
oxalate. *     When  the  precipitate  is  ignited,  a  mixture  of  CaO 
and  Mn2O3  is  obtained.     It  is  weighed,  and  the  manganese  in 
it  determined  volumetrically.f 

The  iron  and  manganese  are  separated  as  directed  p. 


*  Classen,  Zts.  Anal.  Ch.,  16,  318. 

f  Classen,  Quant.  Anal.,  4th  ed.,  p.  128. 


156  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  manganese  finally  precipitated  as  sulphide,  and  the  mag- 
nesium in  the  filtrate  as  magnesium  ammonium  phosphate. 
If  magnesium  is  absent,  the  manganese  is  determined  as 
rnangano-mangaiiic  oxide  or  sulphate  (p.  120). 

HEMATITE. 

Constituents :     Ferric    Oxide,    Manganic    Oxide    (Copper    Oxide, 
Alumina,  Lime,  Magnesia),  Phosphoric  Acid,  Sulphuric  Acid. 

The  iron,  manganese,  and  calcium  are  determined  as 
above.  If  copper  is  present,  it  is  first  separated  from  the 
other  metals  by  first  submitting  the  double  oxalate  solution 
to  a  very  weak  current.  If,  in  addition  to  iron  (copper, 
if  present)  and  manganese,  phosphoric  and  sulphuric  acids 
are  to  be  determined,  the  metals  are  converted  into  double 
oxalates,  and  iron  and  manganese  completely  removed  (see 
separation  of  Iron  and  Manganese,  p.  118) ;  the  two  acids 
may  now  be  determined  in  the  solution  entirely  free  from 
manganese.  If  only  one  acid  is  to  be  determined,  the  whole 
filtrate  can  be  used ;  otherwise  it  is  diluted  to  a  known 
volume,  and  aliquot  portions  taken  for  analysis.  In  the 
determination  of  either  acid,  the  solution  is  first  acidified 
with  hydrochloric  acid,*  and  then  treated  either  with  barium 
chloride,  or  with  one-third  its  volume  of  ammonia,  and  mag- 
nesia mixture.  About  1  gm.  of  the  mineral  is  needed  for 
the  determination  of  sulphuric  and  phosphoric  acids. 

If  alumina,  as  well  as  phosphoric  acid,  is  present  in 
hematite  (its  presence  is  shown  by  a  white  turbidity  f  of 

*  If  the  acid  carbonates  produced  from  the  oxalates  are  not  decomposed, 
small  hard  crystals  of  acid  carbonates  are  precipitated  together  with  am- 
monium magnesium  phosphate.  These  crystals  are  difficultly  soluble  in 
ammonia,  and  may  make  the  results  too  high. 

t  A  turbidity  often  appears  when  the  solution  is  first  heated,  caused  by 
the  driving  off  of  ammonium  compounds. 


DETERMINATION    OF    IRON,    MANGANESE,    ETC.  157 

aluminium  phosphate  and  hydroxide  in  the  solution  under- 
going electrolysis),  the  manganese  must  always  be  converted 
into  sulphide.  The  iron-free  solution  is  boiled  to  decompose 
hydrogen  ammonium  carbonate,  tartaric  acid  or  a  solution  of 
a  tartrate  added  till  the  precipitate  of  aluminium  hydroxide 
disappears,  and  the  weakly  ammoniacal  solution  precipitated 
hot  with  ammonium  sulphide. 

The  green  manganous  sulphide  is  determined  as  hereto- 
fore directed.  The  phosphoric  acid  may  be  determined  with 
magnesia  mixture,  in  the  filtrate  from  the  manganese  sul- 
phide. 

To  determine  sulphuric  acid  in  presence  of  alumina, 
iron  and  manganese  are  removed,  by  electrolysis,  from  a 
separate  portion,  the  solution  is  -poured  off,  the  ammonium 
carbonate  decomposed  by  heat,  the  solution  acidified  with 
hydrochloric  acid,  and  the  sulphuric  acid  determined  with, 
barium  chloride. 


Determination  of  Iron,  Manganese,  Copper,  Calcium,  Magnesium, 
Phosphoric  Acid,  and  Sulphuric  Acid. 

The  method  of  determining  iron,  manganese,  etc.,  in  the 
same  solution  has  already  been  given.  If  it  is  desired  to 
determine  magnesium  and  phosphoric  and  sulphuric  acids, 
in  the  filtrate  from  manganese  peroxide,  it  is  diluted  to  a 
known  volume,  magnesium  is  determined  in  an  aliquot  part 
with  ammonium  phosphate,  and  phosphoric  and  sulphuric 
acids  in  two  other  portions. 


158  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

LIMONITE. 

Constituents :  Ferric  Hydroxide,  together  with  Manganese  Oxide 
(Lime,  Magnesia),  Phosphoric  Acid,  Sulphuric  Acid,  Silica, 
and  Gangue. 

The  analysis  may  be  conducted  like  those  of  hematite 
and  spathic  iron  ;  but  care  must  be  taken,  at  the  outset,  to 
convert  the  silica  into  the  insoluble  modification  by  evapo- 
rating the  solution,  and  drying  the  residue. 

CLAY    IRON-ORE, 
Constituents:   Iron  Oxide,  Alumina,  Manganese,  and  Water. 

The  mineral  is  digested  with  concentrated  hydrochloric 
acid  till  it  is  completely  decomposed,  the  insoluble  residue  is 
filtered  off,  the  filtrate  evaporated  to  remove  free  acid,  the 
residue  dissolved  in  water  with  a  few  drops  of  hydrochloric 
acid,  and  the  iron  separated  from  aluminium  and  manganese 
as  directed  p.  124. 

BOG    IRON-ORE. 

Mixture  of  Ferric  Hydroxide  with  Ferrous  and  Ferric  Silicates, 
Manganese,  Alumina,  Copper,  Calcium,  Magnesium,  Sulphuric 
Acid,  Phosphoric  Acid,  Arsenic  Acid,  Organic  Matter,  and 
Gangue. 

The  analysis  of  the  mineral  is  easily  understood  from  the 
foregoing. 

Arsenic  and  copper  are  best  determined  by  eliminating 
the  former  as  chloride,  as  directed  p.  142.  and  precipitating 
the  copper  with  hydrogen  sulphide  in  the  greatly  diluted 
residue  left  in  the  distillation  flask.  The  copper  sulphide  is 
dissolved  in  nitric  acid,  and  determined  electrolytically  as 
directed  p.  89. 


CHROME    IRON    ORE.  —  PSILOMELANE.  159 

CHROME    IRON    ORE. 

Constituents :    Chromium    Oxide,    Ferrous    and    Ferric    Oxides, 
Alumina,  Manganese,   Calcium,  Silica. 

The  finely  powdered  mineral  is  fused  for  a  long  time  with 
sodium  carbonate  and  potassium  chlorate,  and  the  fused  mass 
extracted  with  water.  The  residue  contains  oxides  of  iron, 
manganese,  calcium,  magnesium,  and  aluminium,  and  traces 
of  chromium  and  silica ;  the  solution,  chromic  acid,  silica, 
and  some  alumina  and  lime.  The  residue  is  dissolved  in 
hydrochloric  acid,  the  solution  evaporated  to  dryness  to 
separate  silica,  the  residue  treated  with  water  and  a  little 
hydrochloric  acid,  and  filtered.  The  metals  in  the  filtrate 
are  converted  into  double  oxalates.  If  manganese  is  present, 
the  precipitate  of  calcium  oxalate  must  be  treated  as  directed 
p.  155.  The  filtrate  from  the  calcium  oxalate,  which  contains 
iron,  manganese,  aluminium,  and  chromium,  is  treated  as 
directed  p.  124.  The  aqueous  solution  from  the  fused  mass 
is  evaporated  to  separate  silica,  the  calcium  precipitated  as 
oxalate,  and  the  aluminium  and  chromic  acid  separated  accord- 
ing to  previous  directions. 

Edgar  F.  Smith  recommends  the  use  of  the  galvanic  cur- 
rent for  the  decomposition  of  chrome  iron  ore.  The  process, 
according  to  his  directions,  is  conducted  as  follows :  Thirty 
or  forty  gm.  potassium  hydroxide  are  heated  in  a  nickel 
crucible  until  the  mass  is  in  a  condition  of  quiet  fusion.  The 
chrome  iron  ore  for  decomposition  (about  0.5  gm.)  is  finely 
pulverized,  weighed  on  a  watch-glass,  and  gradually  added, 
with  the  help  of  a  camePs-hair  pencil,  to  the  crucible  contain- 
ing the  fused  alkali.  The  crucible  is  then  covered  with  a 
perforated  watch-glass  and  connected  with  the  anode  of  the 
battery  or  other  source  of  current.  The  kathode  employed  is 
a  thick  platinum  wire,  which  is  plunged  through  the  opening 


160         QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

in  the  watch-glass  into  the  fused  mass.  To  regulate  the  current 
an  amperemeter  (p.  61)  is  inserted,  and  a  switch  is  also  placed 
in  the  circuit,  so  adjusted  as  readily  to  produce  the  reversal  of 
the  current,  which  is  necessary  toward  the  close  of  the  process. 
The  current  strength  must  not  exceed  1  ampere.  After  about 
30  minutes  the  current  is  reversed  by  the  switch,  so  that  the 
crucible  becomes  the  kathode,  and  the  platinum  wire  the  anode. 
The  object  of  this  reversal  is  to  oxidize  completely  the  last 
traces  of  the  mineral,  minute  portions  of  which  may  have  been 
protected  by  metallic  iron  which  had  been  deposited  by  the 
current.  After  the  current  has  acted  in  this  direction  for  10 
minutes,  the  decomposition  is  complete.  The  fused  mass 
of  course  contains  the  chromium  as  chromate. 

The  author  pursued  similar  researches  some  years  since,, 
and  can  confirm  Smith's  results. 

FSILOMELANE. 

Constituents :  Manganous  Oxide,  Copper  Oxide,  Ferric 
Oxide,  Nickel  Oxide,  Cobalt  Oxide,  Alumina,  Lime, 
Potash,  Soda,  and  Lithia. 

Determination  of  Manganese,  Copper,  Iron,  Aluminium, 
Nickel,  Cobalt,  and  Calcium. 

A  weighed  portion  of  the  mineral  is  dissolved  in  hydro- 
chloric acid,  evaporated  to  dryness,  dissolved  in  water  with 
a  few  drops  of  hydrochloric  acid,  converted  into  double 
oxalates,  calcium  oxalate  filtered  off,  and  the  calcium  and 
manganese  in  the  precipitate  determined  as  directed  p.  155. 
In  the  filtrate,  the  copper  is  first  determined  electrolyti- 
cally  (p.  89).  After  the  precipitation  of  the  copper  is- 
complete,  the  solution,  which  contains  the  other  metals, 
is  decanted  from  the  copper  precipitate,  and  is  then  again 
submitted  to  electrolysis  for  the  precipitation  of  iron,  co- 


PSILOMELANE.  161 

bait,  nickel,  and  manganese,  the  latter  as  dioxide  at  the 
positive  electrode.  After  the  electrolysis  is  completed,  the 
solution  is  decanted  from  the  precipitated  metals,  and  the 
remaining  manganese  completely  precipitated,  according  to 
directions  given  on  p.  120.  If  only  the  weight  of  nickel 
and  cobalt  together  is  desired,  the  precipitate  containing  the 
three  metals  is  dissolved  in  hydrochloric  acid,  and  the  iron 
determined  by  titration  with  potassium  permanganate  as 
directed  p.  115.  Otherwise  the  cobalt  and  nickel  must  first 
be  separated  from  the  iron.  The  precipitate  of  the  metals  is 
dissolved  in  hydrochloric  acid,  the  acid  removed  by  evapora- 
tion, the  residue  oxidized  with  hydrogen  peroxide  or  bromine 
water,  dissolved  in  water  with  a  few  drops  of  hydrochloric 
acid,  and  the  metals  converted  into  double  oxalates  by  addi- 
tion of  potassium  oxalate  in  slight  excess.  From  the  boiling 
solution,  which  should  have  a  volume  of  80-100  cc.,  the 
cobalt  and  nickel  are  precipitated  as  oxalates  by  concen- 
trated acetic  acid.  A  great  excess  of  acetic  acid  must  be 
used,  and  the  solution,  after  the  filtrate  has  subsided,  must 
be  tested  with  the  reagent  for  a  further  precipitate.  The 
filtrate  from  the  cobalt  and  nickel  oxalates  contains  all  the 
iron  as  potassium  iron  oxalate.* 

The  precipitate  of  nickel  and  cobalt  oxalates  is  washed 
with  a  mixture  of  equal  parts  of  alcohol,  acetic  acid,  and 
water,  and,  after  drying  to  remove  acetic  acid  and  alcohol,  is 
dissolved  on  the  filter  with  hot  water  containing  potassium 
and  ammonium  oxalates.  The  solution  is  electrolyzed  as 
directed  p.  81.  The  sum  of  nickel  and  cobalt  is  determined, 
the  metals  dissolved  in  hydrochloric  acid,  evaporated  to  dry- 
ness,  the  residue  dissolved  in  a  few  drops  of  water,  potassium 
hydroxide  added  in  slight  excess,  and  the  resulting  precipi- 

*  Classen,  Zts.  Anal.  Ch.,  18, 189. 


162         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

tate  dissolved  in  concentrated  acetic  acid.  The  cobalt  is 
precipitated  with  a  saturated  solution  of  potassium  nitrite 
acidified  with  acetic  acid.  The  precipitate,  after  standing 
twenty-four  hours,  is  filtered  off,  washed  with  potassium 
nitrite,  and  dissolved  in  hydrochloric  acid,  the  solution  is 
evaporated  to  dryness,  and  the  residue  converted  into  the 
double  oxalate,  and  electrolyzed.  The  nickel  is  determined 
by  difference.  The  nickel  may  also  be  determined,  instead 
of,  or  in  addition  to,  the  cobalt,  by  precipitating  nickel  with 
potassium  hydroxide,  in  the  filtrate  from  the  cobalt  potas- 
sium nitrite,  filtering,  dissolving  in  hydrochloric  acid,  and 
separating  nickel  electrolytically  as  directed  p.  82. 

To  determine  the  iron  in  the  filtrate  from  cobalt  and 
nickel  oxalates,  the  alcohol  and  acetic  acid  are  completely 
removed  by  evaporation,  the  residue  dissolved  in  water,  and 
the  iron  electrolytically  deposited  from  the  solution  of  the 
double  oxalate  (p.  78). 

Determination  of   Potassium,  Sodium,  Lithium,  Calcium,  and 
Magnesium. 

The  mineral  is  dissolved  in  hydrochloric  acid,  evaporated 
to  remove  acid,  and  treated  with  an  excess  of  ammonium 
oxalate.  The  filtrate  from  calcium  oxalate  is  electrolyzed, 
iron,  nickel,  cobalt,  and  copper  separating  as  metals,  man- 
ganese as  dioxide,  and  aluminium  as  hydroxide.  The  filtrate 
from  the  manganese  dioxide  and  aluminium  hydroxide  con- 
tains only  alkalies,  magnesium,  and  a  little  manganese.  It  is 
boiled  to  'remove  the  hydrogen  ammonium  carbonate  formed 
by  the  electrolytic  decomposition  of  ammonium  oxalate,  con- 
centrated to  about  50  cc.,  heated  to  boiling,  and  at  least  an 
equal  volume  of  concentrated  acetic  acid  added.  The  pre- 
cipitate consists  of  manganese  and  magnesium  oxalates.  It 


SPHALERITE    (ZINC    BLENDE).  163 

is  filtered  off,  washed  with  a  mixture  of  equal  volumes  of 
alcohol,  acetic  acid,  and  water,  and  ignited.  The  residue  is 
MgO  +  Mn2O3.  It  is  weighed,  dissolved  in  hydrochloric 
acid,  and  the  manganese  determined  by  electrolysis  as 
dioxide  (p.  84). 

The  alkalies  are  determined  in  the  filtrate  from  the  man- 
ganese and  magnesium  oxalates.  It  is  evaporated  to  dryness, 
the  ammonium  salts  removed  by  gentle  ignition,  the  residue 
dissolved  in  water,  the  solution  filtered,  and  evaporated  to 
dryness  after  addition  of  a  little  hydrochloric  acid.  The 
residue  is  washed  into  a  small  stoppered  flask  with  absolute 
alcohol,  an  equal  volume  of  water-free  ether  added,  and 
allowed  to  stand  twenty-four  hours.  The  solution  is  then 
filtered  from  the  residue,  the  alcohol  and  ether  evaporated, 
and  the  lithium  chloride  converted  into  sulphate  and  weighed. 

The  residue  of  potassium  and  sodium  chlorides  is  dissolved 
in  water,  and  both  metals  directly  determined  as  directed 
p.  145. 

SPHALERITE    (ZINC    BLENDE). 

Constituents :    Zinc    Sulphide,  also    Determinable    Quantities   of 
Iron,  Manganese,  Copper,  Arsenic,  Antimony,   and   Gangue. 

In  most  cases,  it  is  only  necessary  to  determine  the  zinc. 
The  process  is  then  as  follows :  About  0.5  gm.  of  the  finely 
powdered  mineral  is  digested  with  concentrated  nitric  acid 
till  fully  decomposed,  the  acid  evaporated  off,  and  the  nitrates 
•converted  into  chlorides  by  evaporation  with  hydrochloric 
acid.  The  residue  is  dissolved  in  about  25  cc.  water  and 
10  cc.  hydrochloric  acid,  and  hydrogen  sulphide  passed 
through  the  solution.  The  precipitate  of  sulphides  of  lead, 
copper,  etc.,  is  filtered  off,  washed  with  water  containing 
hydrogen  sulphide  and  hydrochloric  acid,  and  the  filtrate 
evaporated  to  dryness.  The  residue  contains  chlorides  of 


164         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

zinc,  iron,  manganese,  calcium,  and  magnesium.  It  is  dis- 
solved in  water  with  a  little  hydrochloric  acid,  converted 
into  double  oxalates  (p.  78),  the  calcium  oxalate  filtered  offt 
and  the  filtrate  electrolyzed.  Zinc  and  iron  separate  at  the- 
negative  electrode,  and  manganese,  as  dioxide,  at  the  positive. 
The  two  metals  are  weighed,  dissolved  in  hydrochloric  acid, 
and  the  iron  determined  by  titration  with  potassium  per- 
manganate (p.  115). 

It  is  stated  on  p.  116  that  the  precipitation  of  iron  and 
zinc  from  the  same  solution  is  complete  only  when  there  is 
less  than  one-third  as  much  zinc  as  iron,  and  that  it  can  be 
successfully  performed,  in  other  cases,  by  adding  a  weighed 
quantity  of  an  iron  salt  before  the  electrolysis. 

Determination  of  Lead,  Copper,  Arsenic,  Antimony,  Zinc,  Iron, 
Manganese,  and   Gaiigue.  , 

As  when  zinc  alone  is  to  be  determined,  the  mineral  is 
oxidized  with  nitric  acid,  the  gangue  filtered  off,  and  the  acid 
solution  of  chlorides  treated  with  hydrogen  sulphide.  The 
precipitated  sulphides  are  washed  first  with  hydrogen  sulphide 
water  containing  hydrochloric  acid,  and  afterward  with  pure 
hydrogen  sulphide  water. 

The  antimony  and  arsenic  are  separated  from  lead  and 
copper  by  digestion  with  a  concentrated  solution  of  sodium 
sulphide  ;  the  residue  is  washed  with  the  same  solution,  and 
afterward  with  hydrogen  sulphide  solution.  The  sodium 
sulphide  washings  are  added  to  the  solution  for  determina- 
tion of  arsenic  and  antimony,  and  the  hydrogen  sulphide 
washings  separately  collected. 

The  necessary  amount  of  sodium  hydroxide  is  added  ta 
the  sodium  sulphide  solution,  and  the  antimony  and  arsenic 
separated  and  determined  as  directed  p.  140. 


CALAMINE    AND    SMITHSONITE. — ULTKAMAKINE.      165 

The  sulphides  of  lead  and  copper  are  dissolved  in  nitric 
acid,  and  the  metals  determined  as  directed  p.  147. 

Iron,  zinc,  and  manganese  are  determined  according  to 
previous  directions. 

CALAMINE    AND    SMITHSONITB. 

Constituents:  Zinc  (Cadmium),  Copper,  Lead,  Arsenic,  Antimony, 
Iron,  Manganese,  Calcium,  Magnesium,  Silica,  Carbonic  Acid, 
Water. 

•  Zinc  and  the  other  constituents  are  determined  as  already 
•directed.  If  the  mineral  contains  cadmium,  copper  and 
lead  are  first  precipitated  from  the  nitric  acid  solution,  the 
decanted  solution  evaporated  to  dryness,  the  cadmium  nitrate 
converted  into  chloride,  and  cadmium  determined  as  directed 
p.  94. 

ULTRAMARINE. 

Constituents  :    Alumina,  Potassium,  Sodium,  Iron,  Calcium, 
Sulphur,  Silica,   Sulphuric  Acid,   Chlorine. 

A  weighed  portion  of  the  substance  is  dissolved  in  hydro- 
chloric acid,  evaporated  to  dryness  to  separate  silica,  the 
residue  dissolved  in  water  with  a  few  drops  of  hydrochloric 
acid,  filtered  from  the  silica,  the  free  acid  neutralized  with 
ammonia,  and  a  great  excess  of  ammonium  oxalate  added. 
The  calcium  oxalate  is  filtered  off,  iron  and  aluminium  deter- 
mined electrolytically,  the  solution  filtered  from  the  alu- 
minium hydroxide,  evaporated  to  dryness,  the  ammonium 
.salts  removed  by  gentle  ignition,  the  residue  dissolved  in 
water,  and  the  alkalies  converted  into  chlorides  by  evapora- 
tion with  hydrochloric  acid.  Potassium  and  sodium  are 
•determined  as  directed  p.  145. 


166         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

REFINERY     SLAG. 

Constituents  :  Ferrous  and  Ferric  Oxides,  Metallic  Iron,  Copper, 
Aluminium,  Calcium,  Magnesium,  Silica,  Sulphuric  and  Phos- 
phoric Acids. 

A  portion  of  the  substance  (0.5-1  gm.)  is  dissolved  in 
hydrochloric  acid,  evaporated  to  remove  silica,  the  residue 
dissolved  in  hydrochloric  acid,  evaporated  to  remove  free 
acid,  and  the  metals  converted,  as  usual,  into  double  oxalates. 
Calcium  oxalate  is  filtered  off,  and  the  manganese  in  the 
precipitate  determined  as  directed  p.  155.  The  copper  is 
separated  by  the  action  of  a  weak  galvanic  current  (p.  89), 
and  the  iron,  manganese,  and  aluminium  separated  in  the 
copper-free  solution  as  directed  p.  123. 

For  the  determination  of  magnesium  and  sulphuric  and 
phosphoric  acids,  see  Hematite,  p.  156. 

To  determine  the  metallic  iron,  about  5  gm.  of  the  finely 
powdered  slag  is  placed  in  a  small  platinum  or  porcelain  dish, 
and  treated  with  an  aqueous  solution  of  copper  sulphate.  A 
quantity  of  metallic  copper  equivalent  to  the  iron  is  precipi- 
tated (CuSO4  +  Fe  =  FeSO4  +  Cu).  The  decomposition 
is  hastened  by  frequent  stirring ;  the  copper  and  undecom- 
posed  slag  are  finally  filtered  off,  washed  thoroughly,  and 
digested  in  the  water-bath  for  a  long  time  with  nitric  acid. 
In  the  solution,  after  filtration,  the  copper  is  electrolytically 
determined,  and  the  quantity  of  iron  calculated  from  it. 

COPPER    AND    LEAD     SLAGS. 

Constituents  :  Copper,  Lead,  Iron,  Manganese,  Barium,  Calcium, 
Magnesium,  Silica,  Sulphuric  Acid,  Sulphur,  and  ordinarily 
small  quantities  of  Arsenic,  Antimony,  Bismuth,  Cobalt, 
Nickel,  and  Zinc. 

The  slag  is  decomposed  by  digestion  with  nitric  acid, 
evaporated  to  dryness,  the  residue  taken  up  with  water  and 


REFINERY   SLAG. — COPPER   AND    LEAD    SLAGS.        167 

a  little  hydrochloric  acid,  and  the  solution  filtered  from  the 
residue  of  silica  and  barium  sulphate,  which  are  separated  as 
usual.  The  calcium  is  separated  by  adding  ammonium  oxa- 
late  in  great  excess ;  the  calcium  and  the  manganese  it  may 
contain  are  determined  as  directed  p.  155.  Copper  is  then 
precipitated  (p.  89),  and  afterward  iron  and  manganese 
(p.  118),  and  magnesium  and  sulphuric  acid  are  determined 
as  directed  p.  156. 

In  the  presence  of  arsenic,  antimony,  etc.,  the  hydrochloric 
acid  solution,  after  separation  of  silica,  is  treated,  first  hot 
and  then  cold,  with  hydrogen  sulphide  gas,  and  the  precipi- 
tated sulphides  are  washed  with  hydrogen  sulphide  water, 
and  treated  with  a  concentrated  solution  of  sodium  sulphide. 
The  insoluble  sulphides  of  lead,  copper,  etc.,  are  washed  first 
with  sodium  sulphide,  and  then  with  hydrogen  sulphide 
(see  p.  154),  and  antimony  and  arsenic  are  separated  in  the 
solution  as  directed  on  p.  140. 

The  residue  of  lead  sulphide,  etc.,  is  digested  with  nitric 
acid  till  thoroughly  decomposed,  and  lead  and  copper  sepa- 
rated from  the  solution  as  directed  p.  130.  The  nitric  acid  is 
evaporated  off,  and  bismuth  nitrate  is  converted  into  chloride, 
and  determined  as  directed  p.  91. 

The  solution  filtered  from  the  hydrogen  sulphide  precipi- 
tate, which  contains  iron,  manganese,  etc.,  is  evaporated 
almost  to  dryness  to  remove  hydrogen  sulphide  and  most  of 
the  hydrochloric  acid,  and  the  metals  finally  converted  into 
double  oxalates.  Calcium  oxalate  is  filtered  off,  and  the 
precautions  described  on  p.  155  are  observed  in  its  deter- 
mination. By  electrolysis  of  the  filtrate,  iron,  cobalt,  nickel, 
and  zinc  are  obtained  as  metals,  and  manganese,  in  part,  as 
dioxide  ;  magnesium  remains  in  solution.  The  two  latter 
are  determined  as  directed  p.  157. 

The  iron,  cobalt,  etc.,  are  dissolved  in  concentrated  hydro- 


168         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

chloric  acid,  the  solution  evaporated  to  dryness,  the  residue 
dissolved  in  water  with  a  few  drops  of  acetic  acid,  potassium 
oxalate  added  in  sufficient  quantity  to  form  the  double  oxa- 
lates,  the  solution  diluted  to  25-30  cc.,  and  precipitated, 
at  boiling  heat,  with  concentrated  acetic  acid  in  great  excess. 
After  standing  about  six  hours  in  a  warm  place,  the  oxalates 
of  cobalt,  nickel,  and  zinc  are  filtered  off,  washed  with  a 
mixture  of  equal  volumes  of  acetic  acid,  alcohol,  and  water, 
and  the  oxalates  converted,  by  very  gentle  heating,  into 
oxides.  The  mixed  oxides  are  dissolved  in  hydrochloric 
acid,  and  zinc  separated  from  nickel  and  cobalt  as  directed 
on  p.  152.  Iron  is  determined,  in  the  nitrate  from  the  oxa- 
lates, as  directed  on  p.  162. 

BLAST    FURNACE,     CUPOLA,     AND     BESSEMER    SLAGS. 

Constituents  :  Ferrous  and  Ferric  Oxides,  Metallic  Iron,  Man- 
ganese, Aluminium,  Copper  Lead,  Zinc,  Calcium,  Magnesium, 
Alkalies,  Silica,  Sulphuric  and  Phosphoric  Acids,  Sulphur 
(as  Calcium  Sulphide). 

The  method  of  analysis  is  so  similar  to  the  foregoing  that 
it  needs  only  brief  mention.  The  slag  is  digested  with 
fuming  hydrochloric  acid,  or  aqua  regia,  till  completely 
decomposed,  the  solution  evaporated  on  the  water-bath  to 
dryness,  the  residue  dissolved  in  water  and  a  little  hydro- 
chloric acid,  and  the  silica  filtered  off.  After  conversion  into 
double  oxalates,  the  calcium  oxalate,  which  may  contain 
manganese,  is  filtered  off  (p.  155),  copper  and  lead  first 
precipitated  (p.  130),  then  iron  and  zinc  with  aluminium  and 
the  rest  of  the  manganese  ;  iron  and  zinc  are  determined  as 
directed  p.  116,  and  manganese,  aluminium,  and  magnesium 
as  directed  p.  156.  The  alkalies  and  sulphuric  and  phos- 
phoric acids  are  determined  as  heretofore  directed. 


ZIRCON. — AESENOPYRITE.  169 

ZIRCON. 
Constituents :    Zirconia,  Iron  Oxide,  Lime,  Silica. 

The  mineral  is  decomposed  by  long-continued  fusion 
with  sodium  carbonate,  the  fused  mass  dissolved  in  hydro- 
chloric acid,  the  solution  evaporated  to  dryness,  the  residue 
taken  up  with  water  acidified  with  hydrochloric  acid,  the 
silica  filtered  off,  and  the  filtrate  treated  with  a  great  excess 
of  ammonium  oxalate.  To  overcome  the  injurious  effect  of 
sodium  chloride,  about  10  gm.  ammonium  oxalate  must  be 
dissolved  by  heating  in  the  solution  diluted  to  about  200  cc. 
Iron  and  zircon  are  separated  as  directed  p.  127.  If  calcium 
is  present,  the  calcium  oxalate  precipitate  is,  of  course,  to  be 
filtered  off  before  electrolysis,  and  determined. 

ARSENOPYRITE. 
Iron,  Arsenic,  Antimony,  Sulphur,  Gangue. 

A  portion  of  the  finely  powdered  mineral  is  oxidized  with 
aqua  regia  till  fully  decomposed,  the  gangue  filtered  off,  and 
the  solution  evaporated  to  dryness.  The  chlorides  are  con- 
verted into  sulphates  by  moistening  and  heating  with  sul- 
phuric acid,  water  is  added,  the  solution  heated  to  70°-80°, 
and  hydrogen  sulphide  passed  till  it  has  cooled  completely. 
After  standing  some  twelve  hours  at  a  moderate  heat,  the 
sulphides  of  arsenic  and  antimony  are  filtered  off,  and  sepa- 
rated as  directed  p.  140. 

To  determine  the  iron,  the  hydrogen  sulphide  is  driven 
off  from  the  solution,  which  is  then  treated  as  directed 
p.  78. 


170         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

CHALCOPYRITE     (COPPER    PYRITES). 

Constituents  :    Copper,  Iron,   Sulphur,   Gaiigue. 

The  mineral  is  oxidized  with  nitric  acid,  the  gangue 
filtered  off,  and  copper  precipitated  in  the  filtrate  (p.  89). 

To  determine  iron,  nitric  acid  is  removed  by  evaporation, 
concentrated  hydrochloric  acid  added,  the  solution  again 
evaporated,  and  finally  iron  is  precipitated,  after  formation 
of  the  double  oxalate,  according  to  directions  on  p.  78. 

Sulphur  may  be  determined  in  the  same  portion  by  pre- 
cipitating sulphuric  acid  with  barium  chloride,  and  removing 
the  excess  of  the  latter  by  careful  addition  of  sulphuric  acid. 
Copper  is  then  separated  from  iron  in  sulphuric  acid  solution,, 
and  the  latter  determined  as  usual. 

As  already  stated  on  p.  91,  copper  cannot  be  precipitated 
from  either  nitric  or  sulphuric  acid  solution  in  the  presence 
of  any  considerable  quantity  of  arsenic  and  antimony  without 
being  contaminated  by  them. 

If  only  the  copper  is  to  be  determined,  the  nitric  acid 
solution  of  the  mineral  is  evaporated  to  dry  ness,  the  residue 
dissolved  in  water  with  a  little  acetic  acid,  and  potassium 
oxalate  added  in  excess.  The  solution  is  filtered  hot  from 
the  gangue,  the  residue  washed  with  water  containing  potas- 
sium oxalate,  and  the  filtrate  brought  to  a  volume  of  about 
50  cc.  After  cooling,  almost  all  the  copper  crystallizes  out 
as  potassium  copper  oxalate  ;  the  rest  is  precipitated  by 
addition  of  much  concentrated  acetic  acid.  The  precipitate 
is  washed  with  a  mixture  of  equal  volumes  of  water,  acetic 
acid,  and  alcohol,  dissolved  in  ammonium  oxalate,  and  elec- 
trolyzed. 

If  arsenic  and  antimony  are  present  in  larger  proportion, 
the  finely  pulverized  mineral  is  mixed  with  four  times  its 


NICKEL    MATTE. — COPPER    MATTE.  171 

weight  of  ammonium  chloride,  and  heated  gently  in  a 
covered  crucible.  Arsenic  and  antimony,  and  the  greater 
part  of  the  iron  are  volatilized  as  chlorides.* 

The  residue  is  dissolved  in  nitric  acid,  and  treated  as. 
before. 

NICKEL    MATTE.      COPPER    MATTE. 

Nickel,  Cobalt,  Zinc,  Iron,  Copper,  Lead,  Arsenic,  Antimony, 
Sulphur,  Gangue. 

The  substance  is  decomposed  with  aqua  regia,  evaporated 
to  dryness,  the  residue  dissolved  in  hydrochloric  acid,  and 
filtered  from  the  gangue.  In  this  solution,  the  metals  pre- 
cipitable  by  hydrogen  sulphide  are  precipitated  by  heating  to 
70°-80°,  and  passing  hydrogen  sulphide  gas  till  the  solution 
becomes  cold.  The  precipitate  is  filtered  off,  washed  first 
with  a  solution  containing  hydrogen  sulphide  and  hydro- 
chloric acid,  then  with  pure  hydrogen  sulphide  solution,  and 
heated  with^a  concentrated  solution  of  sodium  sulphide  as- 
directed  p.  139,  and  the  arsenic  and  antimony  separated 
and  determined  as  directed  p.  140. 

The  sulphides  of  lead  and  copper  left  undissolved  by 
sodium  sulphide  are  digested  with  nitric  acid,  and  deter- 
mined as  directed  p.  130.  The  filtrate  from  the  hydrogen 
sulphide  precipitate  is  evaporated  to  dryness  to  remove 
hydrogen  sulphide  and  hydrochloric  acid,  the  residue  dis- 
solved in  water  with  a  little  acetic  acid,  potassium  oxalate 
added  in  excess,  and  the  solution  of  50-100  cc.  precipitated 
boiling  hot  with  a  great  excess  of  concentrated  acetic  acid 
(at  least  an  equal  volume).  The  precipitate  of  nickel,  cobalt, 
and  zinc  oxalates  is  filtered  off,  washed  with  a  mixture  of 
equal  volumes  of  alcohol,  acetic  acid,  and  water  (p.  161), 

*  Classen,  Zts.  Anal.  Ch.,  18,  388. 


172        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

dried,  and  converted,  by  gentle  ignition,  into  oxides.  The 
residue  is  dissolved  in  hydrochloric  acid,  and  zinc,  cobalt, 
and  nickel  separated,  and  determined  as  directed  pp.  152  and 
161. 

Iron  is  determined,  in  the  filtrate  from  the  mixed  oxalates, 
as  direqted  p.  162. 

^   COPPER  SPEISS,  LEAD  SPEISS. 

Antimony  or  Arsenic  Compounds  of  Iron,  Cobalt,  and  Nickel, 
together  with  Sulphur  Compounds  of  Copper,  Lead,  Silver, 
Bismuth,  Iron,  and  Zinc. 

It  is  best  to  decompose  the  finely  powdered  substance  in 
a  suitable  apparatus  *  with  chlorine  gas,  volatilizing  arsenic, 
antimony,  iron,  and  zinc,  as  chlorides,  and  collecting  them  in 
a  receiver  containing  equal  volumes  of  hydrochloric  and 
tartaric  acids.  The  free  chlorine  is  expelled,  by  heat,  from 
the  solution  in  the  receiver,  and  hydrogen  sulphide  passed 
into  the  still  hot  solution  until  it  cools.  The  sulphides  are 
filtered,  washed,  treated  with  sodium  sulphide,  and  arsenic 
and  antimony  determined  in  the  solution,  as  directed  p.  140. 
The  insoluble  sulphides  of  iron  and  zinc  are  dissolved  in 
hydrochloric  acid,  evaporated  to  dryness,  the  residue  dis- 
solved in  water  with  a  few  drops  of  hydrochloric  acid,  and 
iron  and  zinc  determined  as  directed  p.  116. 

After  the  decomposition  with  chlorine,  the  non-volatile 
chlorides  of  copper,  lead,  silver,  bismuth,  cobalt,  and  nickel, 
and  a  part  of  the  iron  and  zinc,  remain  in  the  bulb.  They 
are  dissolved  in  dilute  hydrochloric  acid,  and  lead,  copper, 
silver,  and  bismuth  precipitated  with  hydrogen  sulphide. 
The  sulphides  are  digested  with  nitric  acid  till  completely 

*  Classen,  Quantitative  Analyse,  4th  ed.    p.  187. 


PYRARGYRITE. — TETRAHEDRITE.  173 

dissolved,  and  copper  and  silver  precipitated  as  metals,  and 
lead  as  peroxide,  by  electrolysis.  Copper  and  silver  are 
separated  as  directed  p.  130,  and  bismuth  from  some  residual 
lead  as  directed  p.  134. 

The  separation  of  cobalt  and  nickel  from  iron  and  zinc  is 
given  on  pp.  152  and  161. 

FYRARGYRITE. 
Silver,  Antimony   (Arsenic),  Sulphur,  Gangue.' 

The  mineral  may  be  decomposed  by  chlorine  gas,  or 
heating  with  anhydrous  sodium  thiosulphate.  In  the  former 
case,  the  chlorides  of  antimony  and  arsenic  (and  sulphur)  go 
into  solution,  while  silver  chloride  remains  in  the  bulb  tube. 
In  the  latter  case,  when  the  fused  mass  is  treated  with  water, 
silver  sulphide  remains  undissolved,  and  may  be  dissolved  in 
nitric  acid,  and  the  silver  deposited,  as  metal,  from  the  solu- 
tion (p.  100). 

To  determine  antimony,  and  separate  it  from  arsenic,  the 
solution  of  sodium  pentasulphide  is  oxidized  with  hydrogen 
peroxide,  evaporated,  and  treated  as  in  the  determination  of 
antimony  in  presence  of  tin  (p.  138). 

TETRAHEDRITE. 

Copper,  Antimony,  Arsenic,  Silver,  Lead,  Iron,  Zinc,  Sulphur, 

Gangue. 

The  mineral  may  be  decomposed  as  heretofore  described. 
When  chlorine  gas  is  used,  the  receiver  contains  chlorides  of 
antimony,  arsenic,  iron,  and  zinc  (and  sulphur)  ;  the  bulb- 
tube,  copper,  lead,  silver,  and  gangue,  with  a  portion  of  the 
iron  and  zinc.  The  metals  are  separated  as  already  described. 


174         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

FURNACE    "SOWS." 

Alloys  of  Iron  (the  principal  constituent),  Copper,  Silver,  Lead, 
Molybdenum,  Vanadium,  Cobalt,  Nickel,  and  Zinc,  with 
Sulphides  and  Phosphides  of  these  Metals,  and  varying 
amounts  of  Carbonic  Acid  and  Silica. 

The  substance  is  best  decomposed  by  chlorine  gas.  The 
quantity  of  iron  is  so  great,  however,  that  two  bulb-tubes 
of  the  most  infusible  glass  should  be  used,  in  the  second  of 
which  is  deposited  most  of  the  iron  chloride.  The  substance 
is  heated  in  a  stream  of  chlorine  as  long  as  iron  chloride 
sublimes  ;  then  it  is  certain  that  all  the  molybdenum 
chloride  will  have  been  carried  over  into  the  receiver,  which 
also  contains  vanadium,  sulphur,  and  phosphorus  chlorides. 
Hydrogen  sulphide  is  passed  into  the  solution  collected  in 
the  receiver  until  the  supernatant  liquid  is  colorless.  The 
precipitate  of  molybdenum  sulphide  is  filtered  off,  washed, 
oxidized  with  nitric  acid,  the  solution  supersaturated  with 
ammonia,  and  molybdenum  oxide  precipitated  by  electrolysis. 

The  filtrate  from  molybdenum  sulphide  contains  vana- 
dium and  iron.  Hydrogen  sulphide  and  hydrochloric  acid 
are  evaporated  off,  double  oxalates  formed,  and  the  two 
metals  separated  electrolytically  as  directed  p.  127.  To 
determine  vanadium  in  the  solution  decanted  from  the  iron, 
it  is  evaporated  to  dryness,  the  ammonium  salts  driven  off 
by  careful  ignition,  and  the  residue  of  vanadium  oxide 
converted,  by  fusion  with  potassium  nitrate,  into  potassium 
vanadate.  The  fused  mass  is  dissolved  in  water,  nitric  acid 
added  not  to  acid  reaction,  then  a  concentrated  solution  of 
ammonium  chloride,  and  then  alcohol  in  the  proportion  of 
one  volume  to  three  of  the  solution.  After  standing  forty- 
eight  hours,  the  ammonium  vanadate  is  filtered  off,  and 
washed  with  a  concentrated  solution  of  ammonium  chloride, 


STIBNITE  (ANTIMONY  GLANCE). — ULLMANITE.     175 

and  then  with  alcohol.  The  salt  is  heated  first  in  the  air, 
then  in  a  stream  of  oxygen,  and  leaves  a  residue  of  pure 
vanadic  acid  which  is  weighed. 

The  chlorides  remaining  in  the  bulb-tube  are  heated  with 
hydrochloric  acid ;  a  residue  of  silver  chloride  and  carbon 
remains.  It  is  heated  with  potassium  cyanide,  the  carbon 
filtered  off,  and  the  silver  determined  by  electrolysis. 

The  methods  of  separation  and  determination  of  the 
metals  in  the  hydrochloric  acid  solution  have  already  been 
given. 

STIBNITE    (ANTIMONY    GLANCE). 

Constituents:    Antimony  and  Sulphur,  and  usually  small 
quantities  of  Iron,  Lead,  Copper,  and  Arsenic. 

The  simplest  method  of  analyzing  the  mineral  is  to  mix 
with  four  or  five  times  its  weight  of  anhydrous  sodium 
thiosulphate,  and  heat  for  a  long  time  in  a  covered  crucible 
(p.  154).  The  fused  mass  is  exhausted  with  water ;  the 
solution  contains  antimony  and  arsenic,  and  is  treated  for 
decomposition  of  sodium  pentasulphide  and  determination  of 
the  two  metals  as  directed  p.  140 ;  the  undissolved  sulphides 
of  lead,  copper,  and  iron  are  oxidized  with  nitric  acid,  and 
the  metals  separated  according  to  foregoing  directions. 

ULLMANITE. 
Antimony,  Nickel,  and  Sulphur. 

The  finely  powdered  mineral  is  decomposed  in  a  stream 
of  chlorine  (p.  172),  all  the  antimony  passing  into  the 
receiver  as  chloride,  and  nickel  chloride  remaining  in  the 
bulb-tube.  The  latter  is  determined  by  dissolving  the  con- 
tents of  the  bulb  in  hydrochloric  acid,  evaporating,  convert- 
ing into  the  double  oxalate,  and  precipitating  by  electrolysis. 


176         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

Antimony  is  precipitated,  as  sulphide,  by  passing  hydro- 
gen sulphide  gas  into   the  solution  in  hydrochloric  and  tar- 
taric  acids,  dissolved  in  concentrated  sodium  sulphide,  the  solu- 
tion diluted  with  water  and  submitted  to  electrolysis  (p.  106). 
If  the  mineral  contains  iron,  it  passes  over,  as  chloride,  into 
the  receiver ;    it  may  be  determined,   in   the   filtrate  from 
antimony  sulphide,  after  supersaturation  with  ammonia,  by 
precipitation  as   sulphide  with   ammonium  sulphide.      The 
sulphide    thus    obtained    is    dissolved,   converted    into    the 
double  oxalate,  and  iron  determined  electrolytically  (p.  78). 

The  analysis  is  made  more  simply  if  the  mineral  is 
decomposed  by  heating  with  sodium  thiosulphate ;  when  the 
proportion  of  antimony  is  large,  it  is  necessary  to  repeat 
the  process  with  the  residual  nickel  sulphide.  Antimony  is 
determined  in  the  aqueous  solution  of  the  fused  mass  as- 
directed  p.  108.  If,  on  treatment  with  hydrogen  peroxide, 
or  addition  of  sodium  monosulphide,  some  nickel  sulphide 
separates,  it  is  added  to  the  principal  portion. 

The  sulphides  of  iron  and  nickel  are  oxidized  with  nitric 
a*cid,  the  nitrates  converted  into  chlorides,  and  the  two 
metals  separated  as  directed  (p.  115). 

BOURNONITE. 
Antimony,  Lead,  Copper  (Iron),  and  Sulphur. 

The  finely  powdered  mineral  is  heated  either  with 
chlorine  or  anhydrous  sodium  thiosulphate,  and  the  analysis 
conducted  as  already  described. 

ZINKENITE. 
Antimony,  Lead  (Silver,   Copper,  Iron),  Sulphur. 

The  mineral  is  most  simply  decomposed  by  heating 
with  anhydrous  sodium  thiosulphate.  After  exhaustion  with 


LIISTN^EITE. — COBALTITE.  177 

water,  the  residue  of  undissolved  sulphides  is  dried,  the 
filter  burnt,  and  fusion  with  thiosulphate  repeated.  Anti- 
mony is  determined  according  to  directions  on  p.  108.  The 
sulphides  of  lead,  silver,  etc.,  are  oxidized  with  nitric  acid ; 
copper  and  silver  precipitated  electrolytically,  and  separated 
as  directed  p.  130.  A  portion  of  the  lead  is  separated,  as 
peroxide,  by  the  electrolysis  of  the  nitric  acid  solution,  and 
is  determined  as  such.  The  rest  is  precipitated  with  hydro- 
gen sulphide,  the  filtrate  neutralized  with  ammonia,  ammo- 
nium oxalate  added,  and  iron  determined  by  electrolysis. 

LINN^BITE. 
Constituents :    Cobalt  and  Sulphur. 

The  analysis  of  this  mineral  is  very  simple.  It  is  dis- 
solved in  aqua  regia,  the  free  acid  evaporated  off,  and 
chlorides  formed  by  repeated  evaporation  with  hydrochloric 
acid. 

The  aqueous  solution  of  the  residue  is  treated  with  an 
excess  of  ammonium  oxalate,  and  cobalt  precipitated  electro- 
lytically (p.  81).  If  iron  is  present,  the  two  metals  are 
separated  as  directed  p.  115. 

In  the  solution  decanted  from  the  metallic  cobalt,  ammo- 
nium carbonate  is  decomposed  by  boiling,  hydrochloric  acid 
is  added,  and  the  sulphur  determined  by  precipitation  with 
barium  chloride. 

COBALTITE. 
Cobalt,  Iron  (Copper,  Antimony),  Arsenic,  and  Sulphur. 

The  mineral  may  be  decomposed  by  heating  with  nitric 
acid,  or  with  sodium  thiosulphate.  If  nitric  acid  is  used, 
the  free  acid  is  evaporated  off,  and  the  nitrates  converted 


178         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

into  chlorides.  In  the  hydrochloric  acid  solution,  arsenic, 
antimony,  and  copper  are  precipitated,  as  sulphides,  by  pass- 
ing hydrogen  sulphide  into  the  hot  solution  till  it  cools  ;  the 
sulphides  are  digested  with  sodium  sulphide,  and  the  solution 
treated  as  directed  p.  140.  The  residue  of  copper  sulphide  is 
dissolved  in  nitric  acid,  and  the  copper  separated  by  elec- 
trolysis (p.  89).  The  nitrate  from  the  hydrogen  sulphide 
precipitate  is  freed  from  hydrogen  sulphide  and  hydro- 
chloric acid,  and  iron  and  cobalt  are  separated  as  directed 
p.  115. 

If  the  mineral  is  heated  with  anhydrous  sodium  thio- 
sulphate,  and  exhausted  with  water,  antimony  and  arsenic 
go  into  solution,  and  are  determined  as  directed  p.  MO. 

The  sulphides  insoluble  in  water  are  dissolved  in  nitric 
acid,  and  copper  first  precipitated  (p.  89)  ;  the  nitrates  are 
then  converted  into  chlorides,  and  cobalt  and  iron  deter- 
mined (p.  115). 

Finally,  arsenic  and  antimony  may  also  be  determined  by 
removing  the  arsenic  first.  The  nitric  acid  solution  is  heated 
with  sulphuric  acid  to  convert  nitrates  into  sulphates.  The 
arsenic  is  driven  off  from  this,  as  chloride,  by  treatment  with 
ferrous  chloride  or  sulphate,  and  distillation  in  a  stream  of 
hydrochloric  acid  (p.  142).  To  determine  antimony,  the 
residue  in  the  flask  is  saturated  with  hydrogen  sulphide,  and 
filtered  ;  the  precipitate  is  washed,  and  treated  with  sodium 
sulphide  (p.  143). 

COBALTITEROUS     ARSENOPYRITB. 
Cobalt,  Iron,  Arsenic,  and  Sulphur. 

The  mineral  is  analyzed  in  the  same  manner  as  co- 
baltite. 


CERUSSITE.— GALENA.  179 

CERUSSITR 
Lead,  Iron,  Calcium,  Carbonic  Acid. 

The  pulverized  mineral  is  dissolved  by  heating  with  nitric 
acid,  and  the  lead  determined,  as  peroxide,  by  connecting 
the  platinum  dish  with  the  positive  pole  of  the  battery 
<p.  97). 

The  solution  decanted  from  the  lead  peroxide  is  evapo- 
rated to  dryness  with  hydrochloric  acid,  the  residue  taken  up 
with  water  arid  a  few  drops  of  hydrochloric  acid,  treated 
with  ammonium  oxalate  in  great  excess,  calcium  oxalate 
filtered  off,  and  iron  determined  electrolytically  in  the  filtrate 
<p.  78). 

GALENA. 

Lead  (Antimony,  Arsenic,  Copper,  Silver,  Gold,  Zinc,  Iron), 
Sulphur,  Gaiigue. 

Galena  rich  in  antimony  is  decomposed  either  by  chlorine, 
or  by  heating  with  anhydrous  sodium  thiosulphate.  When 
decomposed  with  chlorine,  the  receiver  contains  antimony, 
arsenic,  iron,  and  zinc.  These  metals  are  separated  as 
directed  p.  172.  The  chlorides  remaining  in  the  bulb-tube 
are  dissolved  in  hot  dilute  hydrochloric  acid,  and  evaporated 
on  the  water-bath,  with  addition  of  sulphuric  acid,  till  the 
hydrochloric  acid  is  all  driven  off.  The  residue  is  diluted 
with  water,  one-third  its  volume  of  alcohol  added  to  the 
solution,  and  the  lead  sulphate  filtered  off.  In  the  filtrate, 
copper  and  silver  are  precipitated  with  hydrogen  sulphide, 
the  sulphides  oxidized  with  nitric  acid,  and  determined  as 
•directed  p.  130.*  The  filtrate  from  the  hydrogen  sulphide 

*  As  silver  and  gold  are  present  only  in  small  quantities,  they  are 
ordinarily  determined  by  cupellation. 


180        QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

precipitate  is  evaporated,  and  iron  and  zinc  determined  as 
directed  p.  115. 

By  heating  galena  with  sodium  thiosulphate,  and  extract- 
ing with  water,  antimony  and  arsenic  (and  gold)  are  found 
in  the  solution,  and  are  separated  as  already  directed ;  the 
sulphides  of  lead,  silver,  copper,  zinc,  and  iron  remain  undis- 
solved.  The  proportion  of  lead  is  so  great  that  it  cannot 
well  be  determined,  as  dioxide,  in  nitric  acid  solution  ;  it  is 
converted  into  sulphate,  and  the  analysis  completed  as  before. 

PYROMORPHITE. 

Lead  Phosphate  and  Chloride,  sometimes  Sulphate  and 
Arsenate. 

The  finely  pulverized  mineral  is  digested  with  nitric  acid, 
and  evaporated  to  dryness  with  hydrochloric  acid.  The 
residue  is  moistened  with  hydrochloric  acid,  dissolved  in  hot 
water,  the  clear  filtrate  poured  off,  and  the  lead  chloride, 
which  had  crystallized  out,  brought  into  solution  by  repeated 
boiling  with  water.  Lead  and  arsenic  are  precipitated  by 
passing  hydrogen  sulphide  into  the  hot  solution  till  it  cools, 
filtered  hot  after  long  standing,  and  the  precipitate  washed 
and  digested  with  sodium  sulphide.  Arsenic  is  determined 
in  the  solution  as  directed  p.  140.  The  lead  sulphide  is- 
oxidized  with  nitric  acid,  and  lead  determined,  as  peroxide, 
as  directed  p.  97.  Phosphoric  acid  is  determined,  in  the 
usual  way,  in  the  filtrate  from  the  hydrogen  sulphide 

precipitate. 

LEAD    MATTE. 

Lead,  Copper,  Iron  (Silver,  Antimony,  Nickel,  Zinc),  Sulphur. 

If  the  mineral  is  decomposed  by  heating  in  chlorine,  iron 
and  antimony  pass  over  into  the  receiver.  The  analysis  is 
conducted  according  to  directions  for  copper  or  lead  speiss. 


CINNABAR. — BISMUTHINITE.  181 

CINNABAR. 

Constituents :    Mercury,  Manganese,  Copper,  Alumina,  Iron, 
Calcium,  Sulphur. 

The  mineral  is  decomposed  by  heating  with  aqua  regia, 
the  solution  evaporated  on  the  water-bath,  and  the  metals 
-converted  into  nitrates  by  repeated  evaporation  with  nitric 
acid.  Mercury  and  copper  are  precipitated  from  the  nitric 
acid  solution  (pp.  102  and  89),  the  two  metals  weighed,  the 
mercury  driven  off  by  heat,  and  the  residual  copper  oxide 
either  weighed,  or  dissolved  in  nitric  acid,  and  the  metal 
separated  again  by  electrolysis.  The  small  amount  of 
manganese  present  is  precipitated,  as  dioxide,  in  the  elec- 
trolytic process,  and  may  be  weighed  as  such. 

To  determine  iron,  aluminium,  and  calcium,  the  solution 
decanted  from  the  metals  is  evaporated  to  dryness  on  the 
water-bath,  the  nitric  acid  removed  by  repeated  evaporation 
with  hydrochloric  acid,  the  weak  acid  solution  of  the  residue 
treated  with  ammonium  oxalate  in  great  excess,  calcium 
oxalate  filtered  off,  and  iron  and  aluminium  determined  as 
directed  p.  117. 

BISMUTHINITE. 

Bismuth   (Copper,  Lead,  Gold,  Arsenic,  Iron,  Cobalt,  Nickel), 

Sulphur. 

The  mineral  is  decomposed  by  digestion  with  concentrated 
nitric  acid,  the  solution  evaporated,  the  residue  taken  up 
with  hydrochloric  acid,  and  diluted  with  much  water.  The 
bismuth  oxychloride  thus  precipitated  carries  down  some 
copper  with  it.  The  precipitate  is  filtered  and  washed, 
dissolved  in  nitric  acid,  and  copper  precipitated  electrolyti- 
€<illy  from  the  solution.  The  bismuth  is  determined  in  the 


182         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

solution  decanted  from  the  metallic  copper,  by  evaporating 
to  dryness,  converting  into  double  oxalate,  and  electrolyzing 
as  directed  p.  91. 

If  the  bismuth  oxychloride  precipitated  by  water  contains 
a  small  quantity  of  lead,  it  is  precipitated  at  the  same  time 
as  the  copper,  as  peroxide,  at  the  positive  electrode  (p.  96). 

The  solution  filtered  from  the  bismuth  oxychloride  is 
saturated,  first  hot,  and  then  cold,  with  hydrogen  sulphide, 
and  the  precipitate  filtered  off,  and  digested  with  sodium 
sulphide.  The  undissolved  lead  and  copper  sulphides  are 
oxidized  with  nitric  acid,  and  treated  as  directed  p.  130* 
Arsenic  is  determined  in  the  filtrate  as  usual  (p.  140). 

Iron,  cobalt,  and  nickel  are  determined  as  directed  p.  167, 
in  the  filtrate  from  the  hydrogen  sulphide  precipitate,  after 
the  removal  of  hydrogen  sulphide  and  hydrochloric  acid. 

URANINITE     (PITCHBLENDE). 

Urano-Uranic  Oxide,  with  varying  quantities  of  Lead,  Copper, 
Bismuth,  Arsenic,  Antimony,  Sulphur,  Selenium,  Vanadium, 
Iron,  Manganese,  Cobalt,  Nickel,  Zinc,  Calcium,  Magnesium, 
Sodium,  Carbonic  Acid,  Silica. 

The  finely  pulverized  mineral  is  digested  with  concen- 
trated nitric  acid  till  completely  decomposed,  the  solution 
repeatedly  evaporated  with  hydrochloric  acid,  the  residue 
moistened  with  hydrochloric  acid,  dissolved  in  water,  and  the 
silica  filtered  off.  Hydrogen  sulphide  is  passed  into  the  hot 
filtrate  till  it  cools  ;  the  precipitate  is  filtered,  washed,  and 
digested  with  sodium  sulphide. 

Arsenic  and  antimony  (with  selenium)  go  into  solution, 
and  are  separated  as  directed  p.  140.  The  residue  is  digested 
with  nitric  acid,  and  copper  precipitated  as  metal  (p.  89), 
and  lead  as  peroxide  (p.  96),  from  the  solution.  The  nitric 


URANINITE  (PITCHBLENDE).  183 

acid  is  evaporated  off,  the  bismuth  converted  into  double 
oxalate,  and  electrolyzed  (p.  91). 

The  filtrate  from  the  hydrogen  sulphide  precipitate  is 
neutralized  with  ammonia,  and  ammonium  sulphide  added 
drop  by  drop  as  long  as  precipitation  occurs.  The  precipi- 
tate is  filtered  off,  washed,  dissolved  in  hydrochloric  acid 
with  addition  of  hydrogen  peroxide,  and  iron  and  uranium 
precipitated  by  ammonia.  As  the  precipitate  contains  traces 
of  other  metals,  it  is  filtered  off,  washed  with  hot  water, 
dissolved  in  hydrochloric  acid,  and  the  precipitation  with 
ammonia  repeated.  The  precipitate  is  finally  dissolved  in 
hydrochloric  acid,  the  free  acid  driven  off,  and  the  two  metals 
separated  by  electrolysis,  as  directed  p.  125. 

The  filtrate  from  the  precipitate  of  iron  and  uranium, 
which  contains  cobalt,  nickel,  manganese,  and  zinc,  is  con- 
centrated by  evaporation,  treated  with  ammonium  oxalate  in 
great  excess,  and  the  metals  determined  according  to  previous 
directions. 

The  filtrate  from  the  ammonium  sulphide  precipitate 
contains  calcium,  magnesium,  and  alkalies,  which  are  deter- 
mined as  previously  directed. 

If  selenium  is  present,  it  is  determined  as  follows :  A 
larger  portion  of  the  mineral  is  heated  in  a  stream  of  chlorine 
(p.  172) ;  the  receiver  contains,  together  with  chlorides  of  iron, 
bismuth,  arsenic,  and  antimony,  all  the  selenium  as  selenium 
chloride.  The  contents  of  the  receiver  are  heated  to  remove 
free  chlorine  ;  and  the  selenium  is  precipitated,  as  metal,  with 
sulphurous  acid,  or  the  solution  of  an  acid  sulphite. 

Vanadium  is  also  determined  in  a  separate  portion.  The 
finely  powdered  mineral  is  fused  for  a  long  time,  in  a  silver 
crucible,  with  potassium  nitrate  ;  the  fused  mass,  which 
contains  potassium  vanadate,  extracted  with  water ;  and  the 
solution  treated  as  directed  p.  174. 


184         QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

SOFT    LEAD     (CRUDE    LEAD). 

In  addition  to  Lead,  small  quantities  of  Silver,  Copper,  Bis- 
muth, Antimony,  Arsenic,  Cadmium,  Iron,  Zinc,  Cobalt,  Nickel, 
Manganese. 

According  to  the  purity  of  the  metal,  200  to  500  grams 
are  taken.  The  weighed  quantity,  cleaned  and  rolled  into 
thin  plates,  is  digested  with  a  mixture  of  about  250  cc. 
concentrated  nitric  acid,  sp.  gr.  1.4,  and  500-600  cc.  water. 
The  solution  is  hastened  by  careful  heating  on  a  sand  or 
water  bath.  If  the  acid  works  very  actively,  the  flask  is  set 
off  from  the  bath,  but  not  long  enough  for  crystals  of  lead 
nitrate  to  separate  from  the  cooled  solution.  If  there  is  not 
more  than  0.02-0.03  per  cent  of  antimony,  a  perfectly  clear 
solution  is  finally  obtained.  If  the  filtrate  is  turbid  from  the 
presence  of  lead  antimonate,  the  precipitate  is  filtered  off, 
and  washed  thoroughly  with  water  (residue  I.). 

The  nitric  acid  solution  is  transferred  into  a  2-litre 
measuring-flask,  about  170  cc.  of  dilute  sulphuric  acid  are 
added  (1  part  concentrated  sulphuric  acid  and  2  parts 
water),  thus  precipitating  all  the  lead  as  sulphate,  and  the 
flask  is  filled  to  the  mark.  The  contents  of  the  flask  are 
thoroughly  shaken,  the  precipitate  allowed  to  settle,  and  the 
greater  part  of  the  solution  siphoned  off,  taking  care  not  to 
disturb  the  lead  sulphate. 

^,750  cc.  of  the  clear  solution  are  evaporated  till  white 
fumes  of  sulphuric  acid  appear ;  50-60  cc.  of  water  are 
added  after  cooling,  and  the  small  amount  of  lead  sulphate 
that  may  remain  undissolved  is  filtered  off.  As  this  latter 
may  contain  antimony,  it  is  digested  with  concentrated 
sodium  sulphide,  and  the  solution  siphoned  off  (solution  I.). 

The  filtrate  from  lead  sulphate  is  heated  to  about  70°, 
and  hydrogen  sulphide  passed  in  till  it  cools.  When  the 


SOFT   LEAD   (CRUDE   LEAD).  185 

precipitate,  after  long  standing  on  the  sand-bath,  has  com- 
pletely subsided,  it  is  filtered  off,  washed  thoroughly  with 
water  containing  hydrogen  sulphide,  and  digested  with  a 
concentrated  solution  of  sodium  sulphide.  The  residue 
marked  I.  is  also  treated  with  sodium  sulphide,  and  the 
dissolved  portion,  together  with  solution  I.,  added  to  the 
principal  solution.  Antimony  and  arsenic  are  then  separated 
and  determined  as  directed  p.  140. 

The  sulphides  insoluble  in  sodium  sulphide  (copper, 
cadmium,  etc.)  are  digested  with  nitric  acid  till  completely 
oxidized,  and  copper  and  silver  are  separated  from  the 
solution,  as  metals,  by  electrolysis,  and  any  remaining  lead 
as  peroxide.  The  copper  and  silver  are  separated  and 
determined  as  directed  p.  130. 

To  determine  bismuth  and  cadmium,  the  nitric  acid  is 
completely  removed  by  evaporation,  the  residue  dissolved  in 
water  with  a  few  drops  of  dilute  hydrochloric  acid,  potassium 
cyanide  added,  and  the  solution  gently  heated  on  the  water- 
oath ;  the  potassium  bismuth  cyanide  is  filtered  off,  washed 
with  water,  and  dissolved  in  hydrochloric  acid.  The  solution 
is  evaporated  on  the  water-bath,  converted  into  the  double 
oxalate,  and  bismuth  determined  electro lytically  (p.  91). 

Cadmium  can  be  directly  electrolyzed  from  the  solution 
of  cadmium  potassium  cyanide  (p.  94). 

The  filtrate  from  the  original  hydrogen  sulphide  precipi- 
tate, which  contains  zinc,  iron,  manganese,  etc.,  is  evaporated 
to  dryness  on  the  water-bath,  and  the  residue  heated  on  a 
sand-bath  till  no  more  white  fumes  of  sulphuric  acid  are 
seen.  After  cooling,  water  is  added  with  a  few  drops  of 
hydrochloric  acid,  the  solution  is  heated,  and  the  metals 
converted  into  double  oxalates  by  the  addition  of  ammonium 
oxalate  in  excess.  By  the  electrolysis  of  this  solution, 
manganese  is  separated  from  iron,  cobalt,  nickel,  and  zinc 


186        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

(pp.  123  if.);  the  latter  metals  are  dissolved  in  hydrochloric 
acid,  and  zinc  precipitated  from  acetic  acid  solution  by 
hydrogen  sulphide  gas  (p.  152).  The  zinc  sulphide  is  dis- 
solved in  hydrochloric  acid,  and  zinc  determined  by  electro- 
lytic deposition  from  the  double  oxalate  solution  (p.  83). 
The  nitrate  from  zinc  sulphide  is  evaporated  to  dry  ness,  the 
solution  of  the  residue  converted  into  double  oxalates,  and 
iron,  nickel,  and  cobalt  electrolytically  precipitated.  The 
weighed  metals  are  dissolved  in  sulphuric  acid,  and  iron 
titrated  with  potassium  permanganate,  giving  nickel  and 
cobalt  by  difference.  The  quantity  of  the  latter  is  usually 
so  small  that  it  is  not  necessary  to  determine  them  sepa- 
rately ;  it  may  be  done,  however,  as  directed  p.  161. 

In  calculating  the  analysis,  the  space  occupied  by  the  lead 
sulphate  in  the  solution  is  to  be  taken  into  account.  100  gms. 
lead  converted  into  sulphate  occupy  a  space  of  23  cc. ;  200 
gms.,  therefore,  46  cc.  In  calculating,  therefore,  1,750  cc.  are 
to  be  reduced,  not  to  2,000  cc.,  but  to  2,000  —  46  —  1,954  cc., 
or  to  179.12  gms.  lead. 

Crude  lead  is  also  analyzed  by  the  foregoing  method  ; 
10  to  50  gms.  is  a  sufficient  quantity  for  the  analysis. 

« 

HARD    LEAD. 

Hard  lead  is  analyzed  almost  precisely  like  antimony 
alone ;  2  grains  of  the  lead,  beaten  into  thin  plates  or 
otherwise  made  to  expose  a  large  surface,  are  heated  a  long 
time,  in  a  porcelain  crucible,  with  four  or  five  times  the 
weight  of  sodium  thiosulphate  (p.  154)  ;  the  cooled  mass  is 
extracted  with  water,  and  the  lead  sulphide  filtered  off.  To 
remove  the  last  traces  of  antimony,  the  lead  sulphide  is 
washed  three  times  with  5  cc.  concentrated  sodium  sulphide 
solution,  and  then  several  times  with  hydrogen  sulphide 


ANTIMONY. — SPELTER  (CRUDE  ZINC).  187 

water.     Antimony  is  determined,  in  the  filtrate,  exactly  as- 
directed  p.  106. 

ANTIMONY. 

Metallic  antimony  may  be  treated  in  the  same  way  a& 
hard  lead.  After  heating  with  sodium  thiosulphate,  and 
extracting  with  water,  the  undissolved  residue  contains 
sulphides  of  lead,  copper,  silver,  bismuth,  cadmium,  iron, 
zinc,  manganese,  cobalt,  and  nickel ;  and  the  aqueous  solu- 
tion, antimony,  arsenic,  and  tin. 

The  latter  are  separated  as  directed  p.  141,  and  the  other 
metals  as  directed  under  soft  lead. 

SPELTER    (CRUDE    ZINC). 

Zinc  and  determinable  quantities  of   Lead,  Iron,  Cadmium, 
Arsenic,  Antimony,  Tin,   Copper,  and  Manganese. 

In  the  analysis  of  crude  metals,  the  determination  of  the 
impurities  is  of  more  importance  than  that  of  the  metal.  As 
the  quantity  of  other  metals  is  so  small,  it  is  necessary  to 
dissolve  a  large  quantity  of  zinc.  According  to  its  purity, 
25  to  100  gms.  are  taken,  and  dissolved,  in  a  flask,  by  gradual 
addition  of  hydrochloric  acid,  some  zinc,  however,  being  left 
undissolved.  If  the  zinc  comes  in  sticks,  a  stick  may  be 
fastened  to  a  platinum  wire,  and  dipped  partly  into  the 
solution,  and  the  undissolved  zinc  removed,  cleaned,  and 
weighed. 

In  both  cases,  zinc  only  goes  into  solution  ,  the  other 
metals,  with  the  exception  of  arsenic  and  antimony,  being 
left  as  spongy  masses.  It  is  necessary,  however,  to  filter  the 
solution  of  zinc  at  once,  and  to  wash  the  residue.  The 
latter  is  digested  with  nitric  acid,  and  carbon  and  silica,  with 
all  the  tin  oxide  and  small  quantities  of  antimony  (most  of 


188         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

it  was  volatilized  during  the  solution  in  hydrochloric  acid) 
and  lead,  remain  undissolved.  To  determine  the  tin,  the 
residue  is  heated  with  concentrated  hydrochloric  acid,  carbon 
and  silica  are  filtered  off,  the  filtrate  is  evaporated  to  dryness, 
and  the  residue  digested  with  a  concentrated  solution  of 
sodium  sulphide.  The  antimony  and  tin  in  the  filtered 
solution  are  separated  as  directed  p.  138. 

The  nitric  acid  solution  of  the  metals  is  evaporated,  the 
residue  dissolved  in  dilute  hydrochloric  acid,  diluted  with 
water,  and  hydrogen  sulphide  passed  into  the  hot  solution 
till  it  has  thoroughly  cooled.  The  precipitate,  after  settling, 
is  filtered  off,  washed  with  water,  and  digested  with  concen- 
trated sodium  sulphide.  The  sulphides  of  lead,  copper,  etc., 
remain,  are  dissolved  in  nitric  acid,  and  separated  as  in  the 
analysis  of  soft  lead. 

The  filtrate  from  the  hydrogen  sulphide  precipitate  is 
freed  from  hydrogen  sulphide  and  hydrochloric  acid  by 
evaporation  ;  and  the  metals  present,  iron,  zinc,  manganese, 
etc.,  converted  into  double  oxalates,  separated  and  deter- 
mined as  in  the  analysis  of  soft  lead. 

Antimony  and  arsenic  must  be  determined  in  a  separate 
portion  of  zinc,  which  is  dissolved  in  aqua  regia.  The  aqua 
regia  is  evaporated  off,  the  residue  treated  with  concentrated 
hydrochloric  acid,  and  again  evaporated,  and  finally  dissolved 
in  dilute  hydrochloric  acid.  Hydrogen  sulphide  is  passed 
into  the  solution  as  before,  the  sulphides  are  filtered  off  after 
long  standing,  washed  thoroughly  with  water,  and  digested 
Avith  a  concentrated  solution  of  sodium  sulphide.  Antimony 
-and  arsenic  and  tin,  if  present,  are  determined  as  directed 
pp.  140  ff. 


BLISTER   COPPER.  189 

BLISTER    COPPER* 

Copper,  Iron,  Lead,  Silver,  Antimony,  Arsenic,  Bismuth,  Zinc, 
Nickel,  Cobalt. 

Fifty  grams  of  blister  copper  must  be  taken  to  determine 
the  impurities ;  it  is  analyzed  in  two  separate  portions  of 
25  gms.  each.  Each  portion  of  25  gms.  of  bright  copper 
cuttings  is  digested  with  a  mixture  of  about  175  cc.  nitric 
acid  of  1.2  sp.  gr.,  and  200  cc.  water,  till  no  metallic  residue 
is  left ;  and  after  cooling,  whether  the  solution  is  clear  or 
not,  25  cc.  of  concentrated  sulphuric  acid  are  carefully  added. 
The  solution  is  evaporated  on  the  water-bath,  and  heated  on 
the  sand-bath  till  the  excess  of  sulphuric  acid  is  driven  off. 
After  cooling,  20  cc.  nitric  acid  is  added,  the  solution  is 
diluted  with  300  or  400  cc.  water,  and  heated  to  dissolve 
copper  sulphate. 

This  solution  is  treated  with  exactly  enough  f  of  a  titrated 
solution  of  hydrochloric  acid  to  precipitate  the  silver,  and 
allowed  to  stand  twenty-four  hours,  after  which  the  precipi- 
tate (I.)  of  silver  chloride,  lead  sulphate,  antimony  oxide, 
etc.,  is  filtered  off  and  washed  with  water. 

The  filtrate  is  brought  to  a  volume  of  400-450  cc.,  and 
the  copper  separated  by  electrolysis.  For  this  purpose, 
either  a  larger  platinum  dish  is  used,  or  the  platinum  cone 
shown  in  Fig.  42,  p.  68 ;  and  the  current  is  continued  only 
so  long  as  is  necessary  to  remove  the  copper,  as  otherwise 
it  might  be  contaminated  with  antimony  and  arsenic.  If 
the  copper  is  darkened  by  these  metals,  the  process  given 
on  p.  91  must  be  followed. 


*  Process  of  analysis  partly  after  W.  Hampe  ("  Beitrage  zur  Metallurgie 
des  Kupfers"),  Zts.  fur  Berg-,  Hutten-  und  Salinenwesen,  27,  205. 

t  Silver  must  be  previously  determined  in  a  separate  portion  of  25  gms. 


190         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

There  is  usually  a  slight  deposit  of  lead  peroxide  on  the 
positive  electrode  which  is  determined  as  directed  p.  96. 
The  precipitated  copper  contains  bismuth.  To  determine 
the  latter,  the  copper  precipitated  from  both  25-gm.  portions 
is  dissolved  in  about  350  cc.  nitric  acid,  sp.  gr.  1.2 ;  a  great 
excess  of  concentrated  hydrochloric  acid  added,  and  the 
solution  boiled  till  all  the  nitric  acid  is  driven  off.  It  is 
evaporated  on  the  water-bath  till  the  residue  has  a  brown 
color,  and  then  poured  into  a  large  quantity  of  boiling  water 
to  separate  the  bismuth  as  oxy chloride.  The  bismuth  oxy- 
chloride  is  generally  contaminated  with  some  basic  copper 
salt.  If  the  color  shows  the  quantity  of  the  latter  to  be 
considerable,  the  precipitate,  after  standing  twenty-four 
hours,  is  filtered  off,  dissolved  again  in  concentrated  nitric 
acid,  diluted  with  water,  and  copper  precipitated  electro- 
lytically  (p.  89). 

The  bismuth  is  determined  in  the  solution  by  evaporating 
off  the  nitric  acid,  converting  into  the  double  oxalate,  and 
electrolyzing  (p.  91). 

The  solution  siphoned  off  from  the  main  portion  of  the 
copper  is  evaporated  to  dryness,  and  the  sulphuric  acid  set 
free  by  the  precipitation  of  copper  removed  by  heating  on 
the  sand-bath,  so  that  the  residue  contains  only  traces  of  acid. 
After  cooling,  it  is  dissolved  in  hydrochloric  acid  and  water, 
any  silica  from  the  glass  vessels  filtered  off,  and  hydrogen 
sulphide  passed  into  the  solution,  heated  to  70°-80°,  till  it  is 
thoroughly  cool.  The  precipitate,  which  consists  mostly  of 
arsenic  and  antimony,  is  filtered  off  after  long  standing  on 
the  sand-bath,  and  washed  ;  the  filtrate,  containing  iron, 
cobalt,  etc.,  is  retained  (II.). 

Another  portion  of  the  antimony  is  in  residue  I.,  which 
was  left  on  the  solution  of  the  blister  copper  in  nitric  acid. 
Both  precipitates  are  digested  with  a  concentrated  solution 


REFINED   COPPER.  191 

of  sodium  sulphide,  filtered,  and  antimony  and  tin  determined 
as  directed  p.  138.  The  sulphides  insoluble  in  sodium  sulphide 
are  oxidized  with  nitric  acid,  silver  (p.  100)  and  lead  (p,  96 ) 
precipitated  from  the  solution,  the  solution  siphoned  off, 
evaporated  to  remove  nitric  acid,  and  bismuth  determined 
electrolytically  (p.  91). 

Solution  II.  filtered  from  the  hydrogen  sulphide  precipi- 
tate, which  contains  iron,  cobalt,  etc.,  is  evaporated  to 
remove  hydrogen  sulphide,  etc.,  and  the  metals  are  deter- 
mined in  the  residue  as  directed  p.  185. 

REFINED    COPPER. 

This  contains,  in  addition  to  the  metals  present  in  blister 
-copper,  cuprous  oxide.  The  metals  are  determined  as  in 
blister  copper.  The  determination  of  the  cuprous  oxide  is 
based  on  the  fact  that  it  reacts  with  a  dilute  neutral  silver 
solution,  with  the  formation  of  metallic  silver  and  basic 
copper  nitrate,  which  precipitate,  and  normal  copper  nitrate, 
which  remains  in  solution. 

3Cu2O  +  6AgNO3  +  3H2O 

=  2Cu2(OH)3N03  +  2Cu(N03)2  +  6Ag. 

The  process  is  as  follows  :  About  2  gms.  silver  nitrate  is 
dissolved  in  100  cc.  water,  and  about  1  gm.  of  the  copper  to 
be  tested  is  added.  When  the  reaction  is  ended  in  the  cold, 
the  precipitate  is  filtered  off,  and  washed  thoroughly  with 
water ;  either  the  copper  or  the  silver  in  it  may  be  deter- 
mined electrolytically.  The  nitric  acid  is  removed  by 
evaporation,  and  copper  and  silver  separated  as  directed 
p.  130.  If  copper  is  to  be  determined,  silver  is  precipitated 
as  silver  chloride  from  the  aqueous  solution  of  the  residue, 


192         QUANTITATIVE    ANALYSIS   BY   ELECTKOLYSIS 

the  excess  of  acid  removed,  and  copper  precipitated,  by 
electrolysis,  from  solution  of  copper  ammonium  oxalate 
(p.  88). 

TIN. 

The   Impurities   are   usually  Copper,  Lead,   Bismuth,  lion,  Zinc, 
Arsenic,  and   Antimony, 

By  oxidation  of  the  metal  with  nitric  acid,  the  tin  is 
completely  converted  into  insoluble  oxide,  while  the  other 
metals  remain,  for  the  most  part,  in  solution.  The  tin  oxide 
contains,  however,  determinable  quantities  of  lead,  copper, 
antimony,  and  arsenic.  The  methods  already  described  are 
used  for  their  separation  ;  the  tin  oxide  is  digested  with  a 
concentrated  solution  of  sodium  sulphide,  or  fused  with 
anhydrous  sodium  thiosulphite  in  a  porcelain  crucible. 

The  insoluble  sulphides  of  copper  and  lead  are  oxidized 
with  nitric  acid,  and  the  solution  added  to  the  principal 
solution  of  the  metals.  The  rest  of  the  process  is  in 
accordance  with  previous  directions. 

BISMUTH. 

Contains  Lead,  Silver,  Copper,  Arsenic,  Iron,  Cobalt,  and 

Nickel. 

A  large  portion  of  the  metal  is  dissolved  in  dilute  nitric 
acid,  the  acid  evaporated,  the  small  quantity  of  silver  present 
removed  by  hydrochloric  acid,  and  the  solution  greatly 
diluted  with  water ;  all  the  bismuth  separates  as  oxychloride, 
with  traces  of  chlorides  of  the  other  metals.  To  separate 
the  oxychloride  from  the  other  metals,  it  is  dissolved,  after 
nitration  arid  washing,  in  hydrochloric  acid,  and  the  precipi- 
tation repeated.  The  methods  of  separating  the  other  metals 
have  been  already  fully  described. 


SILVER. — COMMERCIAL   NICKEL.  193 

SILVER, 
Traces  of    Gold,  also  Lead,  Copper,  Antimony,  and  Arsenic. 

The  gold  remains  undissolved  when  a  large  quantity  of 
silver  is  dissolved  in  nitric  acid  entirely  free  from  hydro- 
chloric acid.  To  determine  copper  and  lead,  the  silver  is 
precipitated  from  the  largely  diluted  solution  by  hydro- 
chloric acid,  the  silver  chloride  filtered  off,  and  copper  and 
lead  separated,  after  removal  of  hydrochloric  acid,  as  directed 
p.  130. 

As  antimony  and  arsenic  can  only  be  present  in  very 
small  quantities,  they  are  determined  in  a  larger  weight  of 
silver.  The  silver  is  precipitated  as  chloride,  and  the  metals 
precipitable  by  hydrogen  sulphide  by  passing  the  gas  into 
the  hot  filtrate.  Antimony  and  arsenic  are  separated  from 
the  other  metals  by  digestion  with  sodium  sulphide,  and 
determined  as  usual  (p.  140). 

COMMERCIAL    NICKEL. 

Nickel,  Copper,  Arsenic,  Antimony,  Iron,  Cobalt  (Carbon, 
Silica,  Sulphur). 

The  nickel  is  dissolved  in  nitric  acid,  the  insoluble  residue 
filtered  off,  the  nitric  acid  removed  by  evaporation,  the  resi- 
due dissolved  in  hydrochloric  acid,  and  hydrogen  sulphide 
passed  to  remove  the  metals  it  will  precipitate.  It  is  best 
to  redissolve  the  sulphides  and  repeat  the  precipitation. 
Antimony  and  arsenic  are  separated  from  copper  by  digest- 
ing the  sulphide  with  sodium  sulphide,  and  determined  as 
usual.  It  is  to  be  noted,  in  determining  antimony,  that  the 
insoluble  residue  (silica,  etc.)  may  contain  antimony,  and 
must  be  tested  for  it. 


194         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

To  separate  cobalt  and  nickel  from  iron,  the  filtrate  from 
the  hydrogen  sulphide  precipitate  is  evaporated  to  dryness, 
the  residue  oxidized  with  hydrogen  peroxide  or  bromine 
water,  and  dissolved  in  water  with  addition  of  acetic  acid. 
The  metals  are  then  converted  into  double  oxalates  by 
addition  of  potassium  oxalate,  and  cobalt  and  nickel  precipi- 
tated by  acetic  acid.  The  two  metals,  and  the  iron  in  the 
filtrate,  are  determined  as  directed  p.  161. 

If  only  iron  is  to  be  determined,  the  three  metals  are 
precipitated  from  the  double  oxalate  solution  by  electrolysis, 
the  weight  ascertained,  and  the  iron  determined  volumetri- 
cally  in  hydrochloric  acid  solution  (p.  115). 

PIG    IRON,     STEEL,     SPIEGEL,    FERROMANGANESE. 

Constituents  :  Iron,  Manganese,  Copper,  Zinc,  Cobalt,  Nickel, 
Chromium,  Aluminium,  Titanium,  Arsenic,  Antimony,  Cal- 
cium, Magnesium,  Silicon,  Phosphorus,  Sulphur,  Carbon. 

If  a  complete  analysis  of  iron  is  to  be  made,  it  is  best  to 
dissolve  a  large  quantity,  dilute  to  a  known  volume,  and  use 
aliquot  parts  of  the  solution.  In  many  cases,  only  copper, 
or  manganese,  or  certain  other  metals  are  to  be  determined. 
The  complete  analysis  will  first  be  described,  and  afterward 
the  special  determination  of  certain  metals.  5  or  10  grams 
of  the  pure  iron,  in  powder  or  turnings,  are  dissolved  in 
hydrochloric  acid  in  a  capacious  platinum  or  porcelain  dish, 
and  the  solution  evaporated  to  dryness.  The  residue  is 
moistened  with  dilute  hydrochloric  acid,  allowed  to  stand  for 
a  time  that  the  acid  may  act,  dissolved  in  water,  and  the 
insoluble  residue  of  graphite,  silica,  and  compounds  of  iron 
with  titanium,  chromium,  phosphorus,  and  carbon,  filtered 
off.  The  precipitate  is  ignited  with  the  filter,  fused  with 
about  its  own  weight  of  a  mixture  of  equal  parts  of  sodium 


PIG   IRON,  STEEL,  SPIEGEL,  FERROMANGANESE.      195 

carbonate  and  potassium  nitrate,  dissolved  in  water  with 
addition  of  hydrochloric  acid,  and  the  solution  evaporated 
on  the  water-bath.  The  residue  is  heated  for  a  short  time  on 
the  sand-bath  to  insure  separation  of  silica,  moistened,  after 
cooling,  with  hydrochloric  acid,  treated  with  water,  heated, 
-and  the  silica  filtered  off,  weighed,  and  tested  for  titanium. 
The  filtrate  contains  chromium,  together  with  the  rest  of 
the  silica  and  titanium,  and  small  quantities  of  iron  and 
aluminium.  To  completely  separate  silica  and  titanium,  the 
solution  is  evaporated  to  dryness,  the  residue  treated  with 
dilute  sulphuric  acid,  and  heated  till  all  the  hydrochloric 
acid  is  driven  off;  water  is  then  added,  silica  filtered  off, 
and  titanic  acid  precipitated  by  long  boiling.  The  filtrate 
from  titanic  acid  is  concentrated  by  evaporation,  the  free 
sulphuric  acid  neutralized  with  ammonia,  iron,  aluminium, 
and  chromium  converted  into  the  double  oxalates,  and 
•chromium  separated  as  directed  p.  124. 

For  the  determination  of  iron,  aluminium,  zinc,  cobalt, 
nickel,  manganese,  copper,  calcium,  and  magnesium,  an  aliquot 
part  of  the  hydrochloric  acid  solution  is  saturated  with 
hydrogen  sulphide,  and  the  precipitate  filtered  off  after  long 
standing  in  the  heat. 

Since  arsenic  and  antimony  are  ordinarily  present  only  in 
very  small  quantities,  the  copper  sulphide  can  usually  be 
oxidized  with  nitric  acid,  and  the  copper  determined  electro- 
lytically.  If  the  precipitated  copper  is  blackened  by  the 
presence  of  antimony  or  arsenic,  it  is  treated  as  directed 
p.  91. 

The  filtrate  from  the  hydrogen  sulphide  precipitate  is 
freed  from  hydrogen  sulphide  and  hydrochloric  acid  by 
evaporation,  oxidized  with  hydrogen  peroxide  or  a  little 
bromine  water  (by  no  means  with  nitric  acid),  dissolved  in 
water  with  addition  of  a  little  acetic  acid,  and  the  metals 


196         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

converted  into  double  oxalates  by  the  use  of  potassium  (not 
ammonium)  oxalate.  The  insoluble  calcium  oxalate  is  filtered 
off,  and  separated  from  the  manganese  precipitated  with  it  as 
directed  p.  155.  The  filtrate  is  diluted  *  with  water,  heated 
to  boiling,  and  an  excess  of  concentrated  acetic  acid  added, 
whereby  all  the  zinc,  cobalt,  nickel,  and  magnesium,  and  a 
portion  of  the  manganese,  are  precipitated  as  oxalates  ;  iron, 
aluminium,  and  the  rest  of  the  manganese  remain  in  solution 
as  double  oxalates.  The  beaker  is  covered,  and  left  standing 
in  a  warm  place  for  six  hours  ;  the  precipitate  is  then  filtered 
off,  washed  with  a  mixture  of  equal  volumes  of  acetic  acid, 
water,  and  alcohol,  and  dissolved,  after  drying,  in  ammonium 
oxalate.  Zinc,  cobalt,  and  nickel  are  separated  from  man- 
ganese and  magnesium  as  directed  p.  167.  The  filtrate  from 
the  oxalates  is  completely  freed  from  alcohol  and  acetic  acid 
by  evaporation,  and  iron,  aluminium,  and  manganese  sepa- 
rated as  directed  p.  123.  As  the  quantity  of  zinc,  cobalt,  etc., 
is  generally  very  small,  it  is  best,  in  order  to  facilitate  the 
separation  of  the  oxalates  and  the  collection  of  the  precipi- 
tate, to  add  about  0.2  gm.  magnesium  in  the  form  of  chloride, f 
so  that  magnesium  oxalate  is  precipitated  with  the  other 
oxalates.  In  this  case,  the  magnesium  in  pig  iron,  if  present 
at  all,  is  determined  in  another  portion,  together  with  some 
other  metal  (e.g.,  copper).  If  magnesium  is  used,  all  the 
manganese  is  found  in  the  precipitate  produced  by  acetic 
acid. 

To  determine  manganese  alone  in  pig  iron,  either  an 
aliquot  part  of  the  hydrochloric  acid  solution,  or  a  separate 
portion  of  0.2-0.5  gm.  iron  may  be  taken,  and  the  determin- 
ation conducted  as  directed  under  Spathic  Iron  Ore,  p.  155. 

*  Fifty  cc.  of  the  dilute  solution  should  contain  0.4-0.5  gm.  iron, 
f  Dissolve  magnesium  oxide  in  hydrochloric  acid,  and  remove  the  free 
acid  by  evaporation. 


DETERMINATION   OF   AWSENIC\AND   ANTIMONY.      197 

If  copper  is  to  be  determined,  the  solution  freed  from  acid 
and  preferably  oxidized  is  treated  with  ammonium  oxalate 
in  great  excess,  and  electrolyzed  as  already  directed.  The 
hydrochloric  acid  solution  may  also  be  precipitated  with 
hydrogen  sulphide,  and  the  copper  determined  in  nitric  acid 
solution  (see  p.  89). 

Determination  of  Arsenic  and  Antimony. 

Since  these  metals  are  present  only  in  very  small  quantity, 
about  10  gms.  pig  iron  are  used  for  their  determination,  and 
digested  with  aqua  regia.  When  solution  is  complete,  the 
aqua  regia  is  removed  by  evaporation,  the  residue  treated 
with  hydrochloric  acid,  and  heated  till  no  nitric  acid  remains. 

The  solution  is  diluted,  heated,  and  hydrogen  sulphide 
passed  into  it  until  it  is  thoroughly  cool  (p.  lt>9) ;  the  precipi- 
tated sulphides  of  arsenic,  antimony,  and  copper  are  filtered 
off,  thoroughly  washed,  and  digested  with  sodium  sulphide. 
The  solution  is  treated  like  one  containing  polysulphides, 
and  arsenic  and  antimony  separated  as  directed  p.  140. 

Determination  of   Phosphorus. 

About  2  gms.  of  iron  is  digested  with  nitric  acid,  sp.  gr. 
1.2,  till  decomposition  is  complete.  If  a  carbonaceous  residue 
is  left,  the  nitric  acid  solution  is  poured  off,  and  the  residue 
heated  with  aqua  regia.  Nitric  acid  and  aqua  regia  are 
completely  removed  by  evaporation  to  dryness,  and  the 
nitrates  converted  into  chlorides  by  repeatedly  moistening 
with  concentrated  hydrochloric  acid,  and  evaporating  to 
dryness.  The  residue  is  treated  with  water,  heated,  and 
the  iron  brought  into  solution  by  the  addition  of  the  least 
possible  quantity  of  hydrochloric  acid.  To  convert  the  iron, 
'etc.,  into  double  oxalates,  six  or  eight  times  the  weight  of 
the  iron,  reckoned  as  oxide,  of  a  mixture  of  1  part  potassium 
oxalate  and  5-6  parts  ammonium  oxalate,  is  dissolved  by 


198         QUANTITATIVE    ANALYSIS -BY    ELECTROLYSIS. 

heating  in  the  solution,  it  is  diluted  to  250-300  cc.,  and 
electrolyzed  at  a  temperature  of  about  80°.  The  heat  is 
maintained  during;  the  reaction;  the  solution  must  by  no 
means  be  heated  to  boiling,  lest  the  iron  scale  off.  The 
solution  is  poured  off  when  the  reduction  is  complete,  and 
phosphoric  acid  determined  as  directed  p.  127. 

Two  grams  of  iron  are  enough  for  the  determination  of 
phosphorus,  even  when  the  percentage  is  small.  If  a  larger 
quantity  is  taken,  it  is  best  to  divide  the  solution,  after 
conversion  into  oxalates,  and  precipitate  in  several  dishes. 
If  not  more  than  2  gms.  iron  are  present,  it  is  all  separated 
in  two  hours  by  a  current  of  20  cc.  oxyhydrogen  gas  per 
minute,  while  the  reduction  of  4  gms.  requires  seven  hours 
because  of  its  poor  conductivity. 

As  it  is  not  necessary  to  determine  the  iron,  it  may  be 
precipitated  just  as  well  in  a  beaker ;  in  this  case,  the 
negative  electrode  is  a  large  piece  of  light  platinum  foil 
which  is  attached  by  a  platinum  wire  to  the  conductor  from 
the  zinc  of  the  battery. 

Determination   of  Sulphur. 

About  2  grams  of  iron  is  oxidized,  with  aqua  regia,  to 
convert  sulphur  into  sulphuric  acid,  and  the  insoluble  resi- 
due filtered  off.  As  a  portion  of  the  sulphur  may  be  left 
in  the  residue,  it  is  fused  with  a  small  quantity  of  a  mixture 
of  sodium  carbonate  and  potassium  nitrate,  the  fused  mass 
dissolved  in  hydrochloric  acid,  and  the  solution  thus  obtained 
added  to  the  other.  The  aqua  regia  is  removed,  the  nitrates 
converted  into  chlorides,  and  the  latter  into  double  oxalates, 
as  already  directed.  After  removing  the  iron  by  electrolysis, 
the  solution  is  poured  off,  boiled  to  remove  ammonia,  acidified 
with  hydrochloric  acid,  and  the  sulphuric  acid  precipitated 
with  barium  chloride. 


TABLES    FOR    CALCULATION    OF    ANALYSES. 


199 


TABLES     FOR    CALCULATION    OF    ANALYSES. 


Atomic 
Weight. 

Found. 

Sought. 

Factor. 

Aluminium    . 

27.04 

A1203 

Al 

0.5304 

Antimony 

120.29 

Sb 

Sb208 

1.19902 

Sb2S3 

1.39879 

Arsenic     .     .     . 

74.9 

As 

As2O8 

1.31962 

As205 

1.53271 

As2S8 

1.64192 

Barium     .     .     . 

136.86 

BaSO4 

Ba 

0.58819 

BaOO8 

Ba 

0.69574 

BaO 

0.77688 

Beryllium      .     . 

9.08 

BeO 

Be 

0.36262 

Bisntuth    . 

207.5 

Bi 

Bi208 

1.11538 

Boron  .... 

10.9 

KBF4 

B 

0.08639 

BA 

0.27613 

Bromine    . 

79.7.6 

AgBr 

Br 

0.42556 

Cadmium  .     .     . 

111.7 

Cd 

CdO 

1.14288 

CdS 

1.28630 

Caesium     . 

132.7 

Calcium    .     .     . 

39.91 

CaO 

Ca 

0-.71433 

CaCO3 

Ca 

0.40006 

CaO 

0.56004 

Carbon     . 

11.97 

CO2 

C 

0.272727 

Ca003 

C02 

0.43995 

Cerium 

141.2 

Chlorine   . 

35.37 

AgCl 

Cl 

0.24729 

Ag 

Cl 

0.32853 

Chromium 

52.45 

Cr208 

Cr 

0.81419 

CrO8 

1.18581 

200 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Atomic 
Weight. 

Found. 

Sought. 

Factor 

Cobalt      .    .    . 

58.60 

Co 

CoO 

L27116 

Copper     .    .    . 

63.13 

Cu 

CuO 

1.25261 

CuS 

1,25309 

Didymium     •     • 

145.0 

Erbium     .     .     . 

166.0 

Fluorine   . 

19.06 

CaF2 

F 

0.48853 

Gold    .... 

196.2 

Au 

Au2O8 

1.12202 

Hydrogen      .     . 

1 

H20 

H 

0,11136 

Iodine.     .     .     . 

126.54 

Agl 

I 

0.54031 

Ag 

I 

1.17546 

Iron     .... 

55.88 

Fe 

FeO 

1.28561 

Fe203 

1.42842 

Lanthanum   .     . 

138.5 

Lead    .... 

206.39 

PbO2 

Pb 

0.86605 

PbO 

0.93303 

PbCl2 

1.16289 

Lithium    .    .     . 

7.01 

LiCl 

Li 

0.165408 

Li20 

0.35370 

Li8PO4 

Li 

0.18156 

Li2O 

0.38824 

LiCl 

1.09764 

Magnesium   .     . 

23.94 

Mg,PA 

Mg 

0.21614 

MgO 

0.36024 

Manganese    .     . 

54.8 

Mn8O4 

Mn 

0.72029 

MnO 

0.93007 

Mn208 

1.03496 

Mn02 

Mn 

0.63192 

MuO 

0.81596 

Mn2O8 

0.90798 

MnS04 

Mn 

0.36383 

MnO 

0.46979 

Mn2O8 

0.52277 

TABLES  FOR  CALCULATION  OF  ANALYSES. 


201 


Atomic 
Weight. 

Found. 

Sought. 

Factor. 

Mercury   .     .     . 

199.8 

Hg 

Hg,0 

1.03994 

HgO 

1.07988 

HgCl 

1.17703 

Hg2S 

1.08003 

HgS 

1.16006 

Molybdenum 

95.9 

MoS3 

Mo 

0.49989 

Nickel  .... 

58.6 

Ni 

NiO 

1.27116 

Niobium   . 

93.7 

Nitrogen  .     .     . 

14.01 

Pt 

N 

0.14411 

NH8 

0.17497 

NH4 

0.18526 

Osmium    . 

195 

Palladium 

106.2 

Phosphorus    . 

30.96 

Mg2P2O7 

P 

0.27952 

P2Q5 

0.63976 

Platinum  .     .     . 

194.43 

Pt 

PtO2 

1.16417 

Potassium 

39.03 

Pt 

K 

0.40129 

K20 

0.48848 

KC1 

0.76495 

K2S04 

0.89389 

Rhodium  .     .     . 

104.1 

Rubidium       .     . 

85.2 

Ruthenium     .     . 

103.5 

Selenium  . 

78.87 

Silicon 

28 

SiO2 

Si 

0.46729 

Silver  .... 

107.66 

Ag 

Ag20 

1.07412 

AgCl 

1.32853 

Sodium 

22.99 

NaCl 

Na 

0.39393 

Na20 

0.53067 

Na2SO4 

1.21488 

Strontium 

87.3 

SrS04 

Sr 

0.47673 

SrO 

0.56389 

202 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Atomic 
Weight. 

Found. 

Sought. 

Factor. 

Sulphur    .     A   , 

31.98 

BaSO4 

s 

0.13744 

»!                             "'? 

S08 

0.34322 

->•*'  '-'•./' 

SO4 

0.41181 

Tantalum"     .     . 

182 

.    Tellurium       .     . 

127.7 

Thallium  .     .     . 

203.7 

T120 

Tl 

0.9623 

Thorium  . 

231.96 

Tin      .... 

117.35 

Sn 

SnO2 

1.27201 

Titanium  .     .     . 

50.25 

TiO2 

Ti 

0.61154 

Tungsten  . 

183.6 

W03 

W 

0.79316 

Uranium  .     .     . 

239.8 

UO2 

U 

0.88249 

U8O8 

1.03916 

Vanadium 

51.1 

V2O5 

V 

0.56154 

Yttrium    .     .     . 

89.6 

Zinc     .... 

64.88 

Zn 

ZuO 

1.24599 

ZnS 

1.49291 

Zircon       .     .     . 

90.4 

Zr02 

Zr 

0.73904 

REAGENTS.  203 


REAGENTS. 


POTASSIUM    OXALATB. 

The  crystallized  potassium  oxalate  of  commerce  always 
contains  de terminable  quantities  of  iron  and  lead.  To  purify 
it,  one  part  of  the  salt  is  dissolved  in  three  parts  of  water  in 
a  porcelain  dish,  and  ammonium  sulphide  is  added  drop  by 
drop,  as  long  as  a  precipitate  forms.  The  solution  is  now 
heated  on  the  water-bath  till  the  precipitate  settles,  and 
filtered  through  a  plaited  filter.  To  decompose  the  slight 
excess  of  ammonium  sulphide,  a  current  of  air  is  conducted 
through  the  solution  till  it  is  perfectly  colorless,  and  no 
longer  gives  a  reaction  with  sodium  nitroprusside.  The 
separated  sulphur  is  allowed  to  settle,  and  the  clear  solution 
siphoned  off. 

AMMONIUM    OXALATE. 

The  same  impurities  are  present  as  in  potassium  oxalate. 
The  salt  is  purified  by  precipitating  the  hot  saturated  solution 
with  ammonium  sulphide.  It  is  heated  over  a  naked  flame  till 
the  precipitate  coheres  together,  and  filtered  hot  by  the  use 
of  a  hot-water  funnel.  The  greater  part  of  the  ammonium 
oxalate  crystallizes  from  the  filtrate  on  cooling.  The  solution 
is  poured  off,  and  the  crystals  dried  by  placing  them  in  a 
funnel  stopped  with  asbestos,  and  connecting  with  a  filter- 
pump. 


204  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

OXALIC    ACID. 

The  impurities  are  similar  to  those  of  the  alkali  oxalates ; 
it  is  purified  by  repeated  recrystallization. 

AMMONIUM    SULPHATE. 

This  salt  is  purified  in  the  same  way  as  ammonium 
oxalate. 

SODIUM    SULPHIDE. 

The  crystallized  sodium  sulphide  of  commerce  is  not  only 
exceedingly  impure,  but  is  not  inonosulphide  at  all,  but 
a  mixture  of  polysulphides  and  sodium  Irydroxide.  The 
presence  of  the  latter  explains  that  of  alumina,  which  is 
always  found  in  abundance.  If  commercial  sodium  sulphide 
is  used,  its  solution  must  first  be  completely  saturated,  without 
access  of  air,  with  hydrogen  sulphide  gas.  It  is  better, 
however,  to  prepare  the  substance,  in  which  case  the  process 
is  as  follows  :  Sodium  hydroxide  purified  by  alcohol  is 
dissolved  in  water  to  a  solution  of  1.35  sp.  gr.  The  solution 
is  divided  into  two  equal  parts,  and  one  half,  with  exclusion 
of  air,  saturated  with  the  purest  possible  hydrogen  sulphide 
gas  till  the  volume  ceases  to  increase.  The  hydrogen  sul- 
phide is  purified  by  passing  it  through  a  wash-bottle  of  water, 
and  several  tubes  filled  with  cotton  or  wadding.  When 
completely  saturated,  the  solution  is  filtered  from  the  pre- 
cipitate formed,  and  mixed  with  the  other  half  of  the  sodium 
hydroxide  solution.  Hydrogen  sulphide  is  again  passed  into 
the  mixture,  with  exclusion  of  air,  and  it  is  filtered  again. 
The  nearly  colorless  filtrate  is  evaporated  in  a  capacious 
platinum  or  porcelain  dish,  over  a  strong  free  flame  as  quickly 
as  possible.  It  boils  without  bumping  if  a  platinum  spiral  is 


REAGENTS.  205 

placed  in  it.  As  soon  as  a  thin  crystalline  pellicle  forms  on 
the  surface,  the  boiling  is  stopped,  and  the  solution  poured 
while  hot  into  small  flasks  with  well-ground  glass  stoppers 
which  must  be  filled  full.  It  is  best  to  completely  exclude 
the  air  by  melted  paraffine.  For  the  separation  of  antimony 
and  tin,  the  solution  should  have  a  sp.  gr.  of  1.22-1.225. 

ALCOHOL. 

The  alcohol  used  for  washing  metals  must  be  free  from 
acid,  and,  as  nearly  as  possible,  absolute.  It  is  left  standing 
in  a  large  flask,  for  twelve  hours,  over  quicklime,  and  then 
distilled  off  on  a  water  or  steam  bath.  The  distillate  must 
leave  no  residue  on  evaporation. 


206  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 


ANALYTICAL    RESULTS. 


There  follow  some  of  the  most  important  of  the  many 
determinations  which  have  been  made  in  the  author's  labora- 
tory within  the  past  few  years,  many  of  them  by  students. 

Determination  of   Iron. 

Solutions  of  the  chloride  and  of  the  sulphate  were 
employed.  The  determination  of  iron  in  crystallized  iron 
ammonium  sulphate  gave  14.21,  14.22,  14.25,  and  14.23  ft 
iron.  Calculated  14.28  J6. 

Determination  of    Cobalt. 

Pure  metallic  cobalt  (prepared  from  cobalt  oxalate)  was 
dissolved  in  hydrochloric  and  in  sulphuric  acid,  converted 
into  the  double  oxalate,  and  electrolyzed.  Results  were, 

99.98,  99.99,  99.97,  99.99  <jo. 

Determination  of   Nickel. 

In  the  similar  determination  of  nickel,  the  results  were, 

99.99,  99.98,  99.97,  and  99.99  fi. 

Determination  of   Zinc. 

Pure  zinc  oxide  (made  from  zinc  oxalate)  was  used, 
dissolved  in  hydrochloric  or  sulphuric  acid.  Results  reck- 


ANALYTICAL    RESULTS. 


207 


oned  to  metallic  zinc  were,  99.96,  99.95,  99.99,  99.98,  and 
100^. 

Determination  of    Manganese. 

Pure  manganomanganic  oxide  (made  by  ignition  of  pure 
manganese  oxalate)  was  dissolved  in  hydrochloric  acid,  the 
solution  evaporated  to  dryness,  and  the  residue  converted 
into  the  double  oxalate  by  the  action  of  potassium  oxalate. 
The  dioxide  was,  in  some  cases,  converted  into  mangano- 
manganic oxide ;  in  others,  into  manganese  sulphate. 

Results  were,  99.99,  99.99,  100.01,  100.02,  99.98,  and 
100.01  Jb  Mn304. 

Determination  of   Copper. 

Weighed  quantities  of  pure  crystallized  copper  sulphate 
were  dissolved  in  water,  and  the  copper  determined  both  in 
ammonium  oxalate  and  in  nitric  acid  solution. 

Results  as  follows  :  — 


1st  Method. 

2d  Method. 

Calculated. 

25.28 

25.28 

25.39 

25.29 

25.20 

- 

25.30 

25.29 

- 

25.28 

25.25 

— 

Determination  of    Bismuth. 

Bismuth  nitrate  was  used,  and  converted  into  the  double 
oxalate.  The  quantity  of  bismuth  in  the  nitrate  was  com- 
pared with  that  found.  The  results  were,  99.95,  99.98,  99.97, 
and  99.99  o. 


208 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Determination  of    Cadmium. 


Cadmium  sulphate  was  employed,  and  the  proportion  of 
metal  determined  by  ordinary  gravimetric  methods  for 
control.  Reckoned  to  the  cadmium  in  the  salt,  the  results- 
were,  99.97,  99.95,  99.99,  and  99.98  56. 


Determination  of    Platinum. 


Solutions  of  platinic  chloride,  ammonium  platinic  chloride, 
potassium  platinic  chloride,  potassium  platinic  bromide, 
ammonium  platinic  bromide,  and  platinum  tetrabromide 
were  used. 


(NH4)2PtCl6. 

Pt. 

Pt  found  (%). 

Pt  calculated 

(%). 

0.9474  gm. 
1.5101    " 

0.4161  gm. 
0.6634    " 

43.92     ) 
43.93     j 

43.92 

K2PtCl6. 

1.3656  gm. 
1.7345    " 
8.2558    " 

0.5438  gm. 
0.6956    " 
3.3110    " 

40.103 
40.104 
40.105 

40.11 

Determination  of  Antimony. 

Pure  water-free  antimony  tersulphide  was  prepared,  and 
weighed  quantities  dissolved  in  sodium  sulphide.  Results, 
71.48,  71.50,  71.49,  71.47,  and  71.54  %  antimony.  Calculated, 
71.49  <f>. 

To  test  the  method  given  on  p.  112,  weighed  quantities 
of  antimony  tersulphide  were  heated  with  anhydrous  sodium 


ANALYTICAL    RESULTS. 


209 


thio sulphate,  and  the  antimony  determined  electrolytically 
alter  oxidation  of  the  polysulphide  with  hydrogen  peroxide. 
Results  were,  71.40,  71.57,  71.54,  71.57,  and  71.64  <fo  Sb. 

Determination  of    Tin. 

In  a  solution  of  absolutely  pure  tin  chloride,  the  tin  was 
determined  gravimetrically  by  conversion  into  tin  oxide. 

For  electrolytic  determination,  tin  was  converted  into  the 
double  oxalate,  and  also  into  the  sulpho-salt,  by  treatment 
with  ammonium  sulphide.  Results  were,  99.99,  99.97,  99.98, 
99.99,  99.95,  99.97,  and  99.98  %  tin. 

Separation   of    Iron  from  Cobalt. 

Pure  metallic  cobalt  and  an  iron  double  salt,  the  iron  in 
which  had  been  previously  determined,  were  used  for  the 
analysis. 

Results  as  follows  :  — 


TAKEN. 

FOUND. 

Cobalt. 

Iron. 

Fe  +  Co. 

Co  +  Fe. 

Fe. 

0  1087 

0.0989 

0.2076 

r 

0.2075 
,0.2080 
0.2072 

0.0989 
0.0992 
0.0989 

Separation  of    Iron  from  Nickel. 

The  materials  used  were  similar  to  those  in  the  series  of 
experiments  just  described. 


2LO  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Results  were  :  — 


TAKEN. 

FOUND. 

Ni. 

Fe. 

Ni  +  Fe. 

Ni  +  Fe. 

Fe. 

0.1059 

0.0989 

0.2048  | 

0.2045 
0.2050 

0.0989 
0.0989 

0.1034 

0.1978 

0.3012  | 

0.3012 
0.3013 

0.1978 
0.1978 

Separation   of    Iron  and  Zinc. 


TAKEN. 

FOUND. 

Fe  (%). 

Zn  (%). 

Fe  (%). 

Zn  (%). 

92.26 

7.74  j 

92.2 
92.21 

(7.8) 
(7.79) 

( 

92.26 

(7.74) 

( 

80 

(20) 

80 

20    I 

80.1 

(19.9) 

( 

79.9 

(20.1) 

Separation  of  Iron   (Cobalt,  Nickel,  and  Zinc)  from 
Aluminium. 

A   mixture    of    the    double    iron    salt    and    alum    was 
taken. 


ANALYTICAL    RESULTS. 


Results   as   follows    (percentages   of    metallic   iron   and 
aluminium  calculated)  :  — 


TAKEN. 

FOUND. 

Fe  (%). 

Al  (%). 

Fe  (%). 

Al  (%). 

r 

95.01 

(5.99) 

95 

5] 

95.02 

(5.98) 

i 

95 

(5) 

r 

80.04 

19.96 

80 

20] 

80.02 

19.98 

1 

79.99 

20.01 

60.01 

39.99 

60 

40  \ 

60.02 

39.98 

( 

60.02 

39.98 

10 

•90  i 

10 

90 

10.02 

89.98 

TAKEN. 

FOUND. 

CALCULATED. 

Fe(NH4)22SO4,6H2O. 

A1203. 

Fe  (%). 

Fe  (%). 

1.9672 
1.3973 

0.15784 
0.17912 

14.21 
14.23 

f       14.28 

Similar  results  were  obtained  in  the  separation  of  cobalt, 
nickel,  and  zinc  from  alumina.* 


*  See  Ber.  d.  Ch.  Ges.,  17,  2467. 


212  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Separation  of  Iron   (Cobalt,  Nickel,  and  Zinc)  from 
Chromium. 


Weighed' quantities  of  pure  ammonium  ferrous  sulphate 
and  chrome  alum  were  used. 


TAKEN. 

FOUND. 

Fe. 

Cr. 

Fe. 

Cr. 

0.2545 

0.0565 

0.2543 

0.0566 

f 

0.2545 

0.0567 

0.2544 

0.0566  { 

I 

0.2544 

0.0566 

Experiments  with  cobalt,  nickel,  and  zinc  gave  equally 
good  results.* 


Separation  of  Iron   (Cobalt,  Nickel,  and  Zinc)  from 
Manganese. 

A  mixture  of  ammonium  ferrous  sulphate  and  man- 
ganese ammonium  sulphate,  (NH4)2  Mn  2SO4  6H2O,  was 
used. 


*  See  Ber.  d.  Ch.  Ges.,  14,  2771. 


ANALYTICAL    RESULTS. 

Results  as  follows :  *  — 


213 


TAKEN. 

FOUND. 

CALCULATED. 

Fe  Salt  (Gms). 

Mn  Salt  (Gms.). 

Fe  (%). 

Fe  (%). 

0.2647 

0.2857 

14.19 

1.1992 

1.011 

14.21 

1.0995 

1.2065 

14.22 

14.28 

1.202 

1.2179 

14.25 

1.205 

1.4065 

14.23 

- 

Separation  of  Iron   (Cobalt,  Nickel,  and  Zinc)  from  Manganese 
and  Aluminium. 

Portions  of  ammonium  ferrous  sulphate,  mangano-man- 
ganic  oxide,  and  aluminium  oxide  were  weighed  out, 
dissolved,  and  diluted  to  a  known  volume  separately,  and 
aliquot  parts  mixed. 

Results  (among  others)  as  follows  :  — 


TAKEN. 

FOUND. 

Uri/ 

A1203. 

Te. 

Mn. 

Fe. 

0.1220 

0.0610 

0.1220 

0.0610 

0.0505 

0.1220 

0.0615 

I 

0.1215 

0.0610 

*  For  further  results  in  the  separation  of  cobalt,  etc.,  see  Ber.  d.  Ch. 
Ges.,  17,2771. 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Iron,   Cobalt,  Nickel,  and  Zinc  from   Chromium. 

There  were  used,  for  the  tests,  ammonium  ferrous  sulphate, 
metallic  cobalt,  nickelous  oxide,  zinc  oxide,  and  pure  chrome 
alum  or  chromium  ammonium  oxalate.  Among  the  results, 
in  the  separation  of  iron  and  chromium,  were  the  following :  — 


TAKEN. 

FOUND. 

Fe. 

Cr. 

Fe. 

Cr. 

0.212 

0.046 

0.212 

0.046 

0.212 

0.0465 

. 

0.2125 

0.0465 

Equally  good  results  were  obtained  in  the  separation  of 
chromium  from  cobalt,  nickel,  and  zinc. 


Iron,   Cobalt,  Nickel,  and  Zinc  from  Manganese,  Chromium, 
and   Aluminium. 


TAKEN. 

FOUND. 

Fe. 

Mn. 

Cr. 

A1203. 

Fe. 

Mn. 

Cr. 

A1203. 

f 

0.122 

0.0305 

0.0465 

0.0505 

0.122 

0.0305 

0.0464 

0.0505 

0.122 

0.0315 

0.046 

0.0510 

I 

0.1215 

0.0305 

0.046 

0.0515 

ANALYTICAL    RESULTS. 


215 


Iron,   Cobalt,   Nickel,   and   Zinc  from   Uranium. 

There  were  used  mixtures  of  ammonium  ferrous  sulphate, 
cobalt,  nickelous  oxide,  and  zinc  oxide,  with  uranous  oxide. 
Some-  results  follow  :  — 


TAKEN. 

FOUND. 

Fe. 

U. 

Fe. 

U. 

0.2155 

0.0532 

0.2155 

0.053 

0.2850 

0.0795 

0.2848 

0.0798 

0.2155 

0.053 

0.2155 

0.0533 

Iron  from  Beryllium  and  Aluminium. 

The  materials   used  were   ammonium   ferrous   sulphate, 
beryllium  oxide,  and  aluminium  oxide. 


TAKEN. 

FOUND. 

Fe. 

Be203. 

A1203. 

Fe. 

A1203. 

0.096 

0.0515 

0.096 

0.028 

0.0505 

0.096 

0.05 

0.0955 

0.0504 

BeO  was  not  determined. 


216 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Iron  from   Zircon. 


TAKEN. 

FOUND. 

Fe. 

ZrO2. 

Fe. 

-0:696 

0;05 

0.097 

- 

0.1 

0.096 

— 

— 

0.096 

Iron  and  Vanadium. 


TAKEN. 

FOUND. 

Fe, 

Vanadic  Acid. 

Fe. 

0.096 

0.05 

0.096 

- 

0.1 

0.096 

— 

- 

0.0955 

- 

- 

0.096 

Iron  from  Manganese,  Aluminium,  and  Phosphoric  Acid. 

There  were  used  ammonium  ferrous  sulphate,  mangano- 
mariganic  oxide,  and  aluminium  phosphate. 


TAKEN. 

FOUND. 

Fe. 

Mu. 

A1203. 

P205. 

Fe. 

Mn. 

[  0.122 

0.069 

0.122 

0.0694 

0.0505 

0.0223 

0.1215 

0.0695 

0.122 

0.069 

Alumina  and  phosphoric  acid  were  not  determined. 


ANALYTICAL    RESULTS. 


217 


Iron  from  Manganese  and   Sulphuric  Acid. 


FE. 

MN. 

S03. 

Taken. 

Found. 

Taken. 

Found. 

Taken. 

Found. 

[0.422 

0.1015 

"  •    ' 

[     0.025 

0.422 

^  04215 

0.101 

^  0.1015 

0.0248 

\     0.024 

[0.421 

0.102 

I     0.0245 

Copper  from  Iron. 
Ammonia  iron  alum  and  copper  sulphate  were  used, 


TAKEN. 

FOUND. 

Cu. 

Fe. 

Cu 

0.1425 

0.14225 

0.1 

0.14225 
0.1425 

—  ..  — 

0.142 

Copper  from  Nickel. 


TAKEN. 

FOUND. 

Cu. 

m. 

Cu. 

0.14225 

0.1 

f      0.14275 
\       0.1425 
[      0.143 

218 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Similar  results  were  obtained  in  the  separation  of  copper 
from  cobalt,  zinc,  etc.* 


Cadmium  from  Zinc. 
Pure  zinc  oxide  and  cadmium  oxide  were  used.f 


TAKEN. 

FOUND. 

ZnO.      Zn. 

CdO.      Cd. 

Zn. 

Cd. 

0.1026  =  0.0847 

0.1169  =  0.1022 

0.0843 

0.1016 

0.1923  =  0.1543 

0.1354  =  0.1184 

0.1543 

0.1188 

Antimony  from  Arsenic. 

For  the  experiment,  weighed  quantities  of  As2O3  were 
converted  into  As2O5,  mixed  with  pure  Sb2S3,  and  separated 
from  a  sodium  sulphide  solution.^: 


TAKEN. 

FOUND. 

Sb2S3  (%). 

As2O3  (%). 

Sb  (calculated  to  Sb2S3)  %. 

67.48 

32.52 

67.56 

50.77 

49.23 

50.97 

42.20 

57.80 

42.09 

32.20 

67.80 

32.23 

*  Classen,  Ber.  d.  Ch.  GesM  17,  2470. 

t  Analyses  performed  by  Mr.  S.  Eliasberg. 

I  Analyses  performed  by  Dr.  Robert  Ludwig. 


ANALYTICAL    RESULTS. 


219 


Antimony  from  Tin.* 


TAKEN. 

FOUND. 

Sb2S3  (%). 

SuO,  (%). 

Sb2S3  (%). 

Su02  (%). 

29.91 

70.09 

30.07 

70.07 

40.95 

59.05 

40.91 

58.99 

60.19 

39.81 

60.28 

39.91 

77.89 

22.11 

77.93 

21.96 

Antimony  from  Arsenic  and  Tin.* 


TAKEN. 

FOUND. 

Sb2S3  (%). 

Su02  (%). 

As203  (%). 
(In  solution  as  As2O5). 

Sb2S3  (%). 

43.62 

31.24 

25.14 

43.70 

32.13 

22.29 

45.58 

32.19 

28.00 

22.92 

49.08 

28.10 

*  (Ludwig.) 


220  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Antimony,  Arsenic,  and  Tin* 


PERCENTAGES  TAKEN. 

PERCENTAGES  FOUND. 

Sb2S3. 

SuO2. 

As203. 

Sb2S3. 

Su02. 

As203. 

31.60 

41.77 

26.63 

31.56 

41.83 

26>88 

26.61 

39.82 

33.57 

26.59 

39.71 

34^09 

19.80 

30.49 

49.71 

19.77 

30.15 

49-76 

—   —  1  -  

(Ludwig.) 


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